Make sure we never pass a negative value to pow, and optimize gamma uniform

src/gl/renderer/gl_postprocess.cpp

@@ -306,13 +306,13 @@ void FGLRenderer::CopyToBackbuffer(const GL_IRECT *bounds, bool applyGamma)
 		mPresentShader->InputTexture.Set(0);
 		if (!applyGamma || framebuffer->IsHWGammaActive())
 		{
-			mPresentShader->Gamma.Set(1.0f);
+			mPresentShader->InvGamma.Set(1.0f);
 			mPresentShader->Contrast.Set(1.0f);
 			mPresentShader->Brightness.Set(0.0f);
 		}
 		else
 		{
-			mPresentShader->Gamma.Set(clamp<float>(Gamma, 0.1f, 4.f));
+			mPresentShader->InvGamma.Set(1.0f / clamp<float>(Gamma, 0.1f, 4.f));
 			mPresentShader->Contrast.Set(clamp<float>(vid_contrast, 0.1f, 3.f));
 			mPresentShader->Brightness.Set(clamp<float>(vid_brightness, -0.8f, 0.8f));
 		}

src/gl/shaders/gl_presentshader.cpp

@@ -60,7 +60,7 @@ void FPresentShader::Bind()
 		mShader.SetAttribLocation(0, "PositionInProjection");
 		mShader.SetAttribLocation(1, "UV");
 		InputTexture.Init(mShader, "InputTexture");
-		Gamma.Init(mShader, "Gamma");
+		InvGamma.Init(mShader, "InvGamma");
 		Contrast.Init(mShader, "Contrast");
 		Brightness.Init(mShader, "Brightness");
 		Scale.Init(mShader, "UVScale");

src/gl/shaders/gl_presentshader.h

@@ -9,7 +9,7 @@ public:
 	void Bind();
 
 	FBufferedUniform1i InputTexture;
-	FBufferedUniform1f Gamma;
+	FBufferedUniform1f InvGamma;
 	FBufferedUniform1f Contrast;
 	FBufferedUniform1f Brightness;
 	FBufferedUniform2f Scale;

wadsrc/static/shaders/glsl/present.fp

@@ -3,14 +3,14 @@ in vec2 TexCoord;
 out vec4 FragColor;
 
 uniform sampler2D InputTexture;
-uniform float Gamma;
+uniform float InvGamma;
 uniform float Contrast;
 uniform float Brightness;
 
 vec4 ApplyGamma(vec4 c)
 {
-	vec3 val = c.rgb * Contrast - (Contrast - 1.0) * 0.5;
-	val = pow(val, vec3(1.0 / Gamma));
+	vec3 val = max(c.rgb * Contrast - (Contrast - 1.0) * 0.5, vec3(0.0));
+	val = pow(val, vec3(InvGamma));
 	val += Brightness * 0.5;
 	return vec4(val, c.a);
 }