- use a non-0 depth bias for sprites.

Hopefully this helps fixing the z-fighting issue with wall sprites in some Blood maps.

source/build/src/engine.cpp

@@ -7646,6 +7646,7 @@ killsprite:
         GLInterface.EnableBlend(false);
         GLInterface.EnableAlphaTest(true);
 		GLInterface.SetClamp(1+2);
+        GLInterface.SetDepthBias(-2, -256);
 
         if (spritesortcnt < numSprites)
         {
@@ -7714,7 +7715,7 @@ killsprite:
         GLInterface.EnableBlend(true);
         GLInterface.EnableAlphaTest(true);
 		GLInterface.SetDepthMask(false);
-	}
+    }
 #endif
 
     vec2f_t pos;
@@ -7855,6 +7856,7 @@ killsprite:
     {
 		GLInterface.SetDepthMask(true);
 		GLInterface.SetClamp(0);
+        GLInterface.SetDepthBias(0, 0);
     }
 #endif
 

source/glbackend/gl_renderstate.h

@@ -55,6 +55,21 @@ enum PRSFlags
 
 };
 
+struct FDepthBiasState
+{
+	float mFactor;
+	float mUnits;
+	bool mChanged;
+
+	void Reset()
+	{
+		mFactor = 0;
+		mUnits = 0;
+		mChanged = false;
+	}
+};
+
+
 struct PolymostRenderState
 {
 	int vindex, vcount, primtype;
@@ -72,6 +87,7 @@ struct PolymostRenderState
 	short matrixIndex[NUMMATRICES] = { 0,0,0,0,0 };
 	PalEntry fullscreenTint = 0xffffff, hictint = 0xffffff, hictint_overlay = 0xffffff;
 	int hictint_flags = -1;
+	FDepthBiasState mBias{ };
 
 	int StateFlags = STF_COLORMASK|STF_DEPTHMASK;
 	FRenderStyle Style{};

source/glbackend/glbackend.cpp

@@ -428,6 +428,19 @@ void PolymostRenderState::Apply(PolymostShader* shader, GLState &oldState)
 			else
 				glDisable(GL_SCISSOR_TEST);
 		}
+		if (mBias.mChanged)
+		{
+			if (mBias.mFactor == 0 && mBias.mUnits == 0)
+			{
+				glDisable(GL_POLYGON_OFFSET_FILL);
+			}
+			else
+			{
+				glEnable(GL_POLYGON_OFFSET_FILL);
+			}
+			glPolygonOffset(mBias.mFactor, mBias.mUnits);
+			mBias.mChanged = false;
+		}
 
 		StateFlags &= ~(STF_CLEARCOLOR | STF_CLEARDEPTH | STF_VIEWPORTSET | STF_SCISSORSET);
 		oldState.Flags = StateFlags;

source/glbackend/glbackend.h

@@ -225,6 +225,20 @@ public:
 	
 	void ReadPixels(int w, int h, uint8_t* buffer);
 
+	void SetDepthBias(float a, float b)
+	{
+		renderState.mBias.mFactor = a;
+		renderState.mBias.mUnits = b;
+		renderState.mBias.mChanged = true;
+	}
+
+	void ClearDepthBias()
+	{
+		renderState.mBias.mFactor = 0;
+		renderState.mBias.mUnits = 0;
+		renderState.mBias.mChanged = true;
+	}
+
 	void SetPaletteData(int index, const uint8_t* data)
 	{
 		palmanager.SetPalette(index, data);