Add sample shading for MSAA (better quality) (#1765)

* Add minimum sample shading rate for MSAA (better quality)
* Change default value to 0.0f

rts/Rendering/GlobalRendering.cpp

@@ -46,6 +46,7 @@ CONFIG(bool, DebugGLStacktraces).defaultValue(false).description("Create a stack
 CONFIG(int, GLContextMajorVersion).defaultValue(3).minimumValue(3).maximumValue(4);
 CONFIG(int, GLContextMinorVersion).defaultValue(0).minimumValue(0).maximumValue(5);
 CONFIG(int, MSAALevel).defaultValue(0).minimumValue(0).maximumValue(32).description("Enables multisample anti-aliasing; 'level' is the number of samples used.");
+CONFIG(float, MinSampleShadingRate).defaultValue(0.0f).minimumValue(0.0f).maximumValue(1.0f).description("A value of 1.0 indicates that each sample in the framebuffer should be independently shaded. A value of 0.0 effectively allows the GL to ignore sample rate shading. Any value between 0.0 and 1.0 allows the GL to shade only a subset of the total samples within each covered fragment.");
 
 CONFIG(int, ForceDisablePersistentMapping).defaultValue(0).minimumValue(0).maximumValue(1);
 CONFIG(int, ForceDisableExplicitAttribLocs).defaultValue(0).minimumValue(0).maximumValue(1);
@@ -162,6 +163,7 @@ CR_REG_METADATA(CGlobalRendering, (
 	CR_IGNORED(forceSwapBuffers),
 
 	CR_IGNORED(msaaLevel),
+	CR_IGNORED(minSampleShadingRate),
 	CR_IGNORED(maxTextureSize),
 	CR_IGNORED(maxFragShSlots),
 	CR_IGNORED(maxCombShSlots),
@@ -274,6 +276,7 @@ CGlobalRendering::CGlobalRendering()
 
 	// fallback
 	, msaaLevel(configHandler->GetInt("MSAALevel"))
+	, minSampleShadingRate(configHandler->GetFloat("MinSampleShadingRate"))
 	, maxTextureSize(2048)
 	, maxFragShSlots(8)
 	, maxCombShSlots(8)
@@ -1660,6 +1663,14 @@ void CGlobalRendering::InitGLState()
 	msaaLevel *= CheckGLMultiSampling();
 	ToggleMultisampling();
 
+	if(msaaLevel > 0 && minSampleShadingRate > 0.0f) {
+		// Enable sample shading
+		glEnable(GL_SAMPLE_SHADING_ARB);
+		if (GLEW_VERSION_4_0) {
+			glMinSampleShading(minSampleShadingRate); 
+		}
+	}
+
 	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
 	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 

rts/Rendering/GlobalRendering.h

@@ -222,6 +222,7 @@ class CGlobalRendering {
 	 * Level of multisample anti-aliasing
 	 */
 	int msaaLevel;
+	float minSampleShadingRate;
 
 	/**
 	 * @brief maxTextureSize