diff --git a/src/gl/renderer/gl_renderstate.cpp b/src/gl/renderer/gl_renderstate.cpp
index 83303d61e..56b319f00 100644
--- a/src/gl/renderer/gl_renderstate.cpp
+++ b/src/gl/renderer/gl_renderstate.cpp
@@ -80,19 +80,44 @@ void FRenderState::Reset()
 	mDstBlend = GL_ONE_MINUS_SRC_ALPHA;
 	mAlphaThreshold = 0.5f;
 	mBlendEquation = GL_FUNC_ADD;
+	mModelMatrixEnabled = false;
+	mTextureMatrixEnabled = false;
 	mObjectColor = 0xffffffff;
 	mVertexBuffer = mCurrentVertexBuffer = NULL;
 	mColormapState = CM_DEFAULT;
+	mSoftLight = 0;
+	mLightParms[0] = mLightParms[1] = mLightParms[2] = 0.0f;
 	mLightParms[3] = -1.f;
 	mSpecialEffect = EFF_NONE;
 	mClipHeight = 0.f;
 	mClipHeightDirection = 0.f;
+	mShaderTimer = 0.0f;
 	ClearClipSplit();
 
 	stSrcBlend = stDstBlend = -1;
 	stBlendEquation = -1;
 	stAlphaThreshold = -1.f;
+	stAlphaTest = 0;
 	mLastDepthClamp = true;
+	mInterpolationFactor = 0.0f;
+
+	mColor.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mCameraPos.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mGlowTop.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mGlowBottom.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mGlowTopPlane.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mGlowBottomPlane.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mSplitTopPlane.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mSplitBottomPlane.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mClipLine.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mDynColor.Set(0.0f, 0.0f, 0.0f, 0.0f);
+	mClipSplit[0] = mClipSplit[1] = 0.0f;
+	mEffectState = 0;
+	activeShader = nullptr;
+	mProjectionMatrix.loadIdentity();
+	mViewMatrix.loadIdentity();
+	mModelMatrix.loadIdentity();
+	mTextureMatrix.loadIdentity();
 }
 
 //==========================================================================
diff --git a/src/gl/shaders/gl_bloomshader.h b/src/gl/shaders/gl_bloomshader.h
index cbc740ab6..b20277a42 100644
--- a/src/gl/shaders/gl_bloomshader.h
+++ b/src/gl/shaders/gl_bloomshader.h
@@ -8,7 +8,7 @@ class FBloomExtractShader
 public:
 	void Bind();
 
-	FBufferedUniform1i SceneTexture;
+	FBufferedUniformSampler SceneTexture;
 	FBufferedUniform1f Exposure;
 	FBufferedUniform2f Scale;
 	FBufferedUniform2f Offset;
@@ -22,7 +22,7 @@ class FBloomCombineShader
 public:
 	void Bind();
 
-	FBufferedUniform1i BloomTexture;
+	FBufferedUniformSampler BloomTexture;
 
 private:
 	FShaderProgram mShader;
diff --git a/src/gl/shaders/gl_lensshader.h b/src/gl/shaders/gl_lensshader.h
index ef0810e4e..7e9ef6392 100644
--- a/src/gl/shaders/gl_lensshader.h
+++ b/src/gl/shaders/gl_lensshader.h
@@ -8,7 +8,7 @@ class FLensShader
 public:
 	void Bind();
 
-	FBufferedUniform1i InputTexture;
+	FBufferedUniformSampler InputTexture;
 	FBufferedUniform1f AspectRatio;
 	FBufferedUniform1f Scale;
 	FBufferedUniform4f LensDistortionCoefficient;
diff --git a/src/gl/shaders/gl_presentshader.h b/src/gl/shaders/gl_presentshader.h
index b54fa2e36..6f4e899bb 100644
--- a/src/gl/shaders/gl_presentshader.h
+++ b/src/gl/shaders/gl_presentshader.h
@@ -8,7 +8,7 @@ class FPresentShader
 public:
 	void Bind();
 
-	FBufferedUniform1i InputTexture;
+	FBufferedUniformSampler InputTexture;
 	FBufferedUniform1f InvGamma;
 	FBufferedUniform1f Contrast;
 	FBufferedUniform1f Brightness;
diff --git a/src/gl/shaders/gl_shader.h b/src/gl/shaders/gl_shader.h
index 75e4b4e5e..52ca59ec7 100644
--- a/src/gl/shaders/gl_shader.h
+++ b/src/gl/shaders/gl_shader.h
@@ -201,6 +201,28 @@ public:
 	}
 };
 
+class FBufferedUniformSampler
+{
+	int mBuffer;
+	int mIndex;
+
+public:
+	void Init(GLuint hShader, const GLchar *name)
+	{
+		mIndex = glGetUniformLocation(hShader, name);
+		mBuffer = -1;
+	}
+
+	void Set(int newvalue)
+	{
+		if (newvalue != mBuffer)
+		{
+			mBuffer = newvalue;
+			glUniform1i(mIndex, newvalue);
+		}
+	}
+};
+
 
 class FShader
 {
diff --git a/src/gl/shaders/gl_shaderprogram.cpp b/src/gl/shaders/gl_shaderprogram.cpp
index 63ce09261..959315a9d 100644
--- a/src/gl/shaders/gl_shaderprogram.cpp
+++ b/src/gl/shaders/gl_shaderprogram.cpp
@@ -52,6 +52,12 @@
 #include "i_system.h"
 #include "doomerrors.h"
 
+FShaderProgram::FShaderProgram()
+{
+	for (int i = 0; i < NumShaderTypes; i++)
+		mShaders[i] = 0;
+}
+
 //==========================================================================
 //
 // Free shader program resources
diff --git a/src/gl/shaders/gl_shaderprogram.h b/src/gl/shaders/gl_shaderprogram.h
index 16684ddff..64a36db49 100644
--- a/src/gl/shaders/gl_shaderprogram.h
+++ b/src/gl/shaders/gl_shaderprogram.h
@@ -6,6 +6,7 @@
 class FShaderProgram
 {
 public:
+	FShaderProgram();
 	~FShaderProgram();
 
 	enum ShaderType
@@ -30,6 +31,9 @@ public:
 	static void PatchFragShader(FString &code);
 
 private:
+	FShaderProgram(const FShaderProgram &) = delete;
+	FShaderProgram &operator=(const FShaderProgram &) = delete;
+
 	static FString PatchShader(ShaderType type, const FString &code, const char *defines, int maxGlslVersion);
 	static void PatchCommon(FString &code);
 
diff --git a/src/gl/shaders/gl_tonemapshader.h b/src/gl/shaders/gl_tonemapshader.h
index 9d427713f..7ec24117b 100644
--- a/src/gl/shaders/gl_tonemapshader.h
+++ b/src/gl/shaders/gl_tonemapshader.h
@@ -8,9 +8,9 @@ class FTonemapShader
 public:
 	void Bind();
 
-	FBufferedUniform1i SceneTexture;
+	FBufferedUniformSampler SceneTexture;
 	FBufferedUniform1f Exposure;
-	FBufferedUniform1i PaletteLUT;
+	FBufferedUniformSampler PaletteLUT;
 
 	static bool IsPaletteMode();
 
diff --git a/src/gl/system/gl_framebuffer.cpp b/src/gl/system/gl_framebuffer.cpp
index 6d56c1b27..ec8b3f599 100644
--- a/src/gl/system/gl_framebuffer.cpp
+++ b/src/gl/system/gl_framebuffer.cpp
@@ -99,6 +99,11 @@ OpenGLFrameBuffer::OpenGLFrameBuffer(void *hMonitor, int width, int height, int
 	// If wglSwapIntervalEXT is called after glBindFramebuffer in a frame the setting is not changed!
 	SetVSync(vid_vsync);
 
+	// Make sure all global variables tracking OpenGL context state are reset..
+	FHardwareTexture::InitGlobalState();
+	FMaterial::InitGlobalState();
+	gl_RenderState.Reset();
+
 	GLRenderer = new FGLRenderer(this);
 	memcpy (SourcePalette, GPalette.BaseColors, sizeof(PalEntry)*256);
 	UpdatePalette ();
diff --git a/src/gl/textures/gl_hwtexture.h b/src/gl/textures/gl_hwtexture.h
index 9b7028f8c..4c00af272 100644
--- a/src/gl/textures/gl_hwtexture.h
+++ b/src/gl/textures/gl_hwtexture.h
@@ -48,11 +48,11 @@ private:
 public:
 
 	static unsigned int lastbound[MAX_TEXTURES];
-	static int lastactivetexture;
-	static int max_texturesize;
 
 	static int GetTexDimension(int value);
 
+	static void InitGlobalState() { for (int i = 0; i < MAX_TEXTURES; i++) lastbound[i] = 0; }
+
 private:
 
 	short texwidth, texheight;
diff --git a/src/gl/textures/gl_material.cpp b/src/gl/textures/gl_material.cpp
index 8e0ab73e6..6ba1fe4b0 100644
--- a/src/gl/textures/gl_material.cpp
+++ b/src/gl/textures/gl_material.cpp
@@ -669,6 +669,12 @@ static FMaterial *last;
 static int lastclamp;
 static int lasttrans;
 
+void FMaterial::InitGlobalState()
+{
+	last = nullptr;
+	lastclamp = 0;
+	lasttrans = 0;
+}
 
 void FMaterial::Bind(int clampmode, int translation)
 {
diff --git a/src/gl/textures/gl_material.h b/src/gl/textures/gl_material.h
index dea0d4cf5..d44a845f6 100644
--- a/src/gl/textures/gl_material.h
+++ b/src/gl/textures/gl_material.h
@@ -263,6 +263,7 @@ public:
 	static FMaterial *ValidateTexture(FTextureID no, bool expand, bool trans);
 	static void ClearLastTexture();
 
+	static void InitGlobalState();
 };
 
 #endif