fixed shader stage collapsing happening in cases where it shouldn't

changelog.txt

@@ -38,6 +38,8 @@ chg: faster map loads by limiting the rendering back-end's frame-rate
 
 chg: on Windows, the upper limit of open stdio file handles was raised from 512 to 2048
 
+fix: shader stage collapsing was happening in cases where it would yield incorrect results
+
 fix: map download improvements
   add: using numeric addresses as fallbacks in case DNS resolves fail
   chg: made connections non-blocking so the engine won't freeze for long when starting a download

code/renderer/tr_shader.cpp

@@ -1721,12 +1721,27 @@ static void CollapseStages()
 		if ( collapse[i].blendA == -1 )
 			CollapseFailure;
 
-		// Check that all colors are pure white on the second stage
+		// Check that all colors are opaque white on the second stage
 		// because the stage iterator can't currently specify
 		// another color array.
 		// Example shader broken without this extra test:
 		// "textures/sfx/diamond2cjumppad"
 		// The ring pulses in and out instead of only out.
+
+		// These cases must always be rejected because they depend
+		// on time elapsed, camera position, entity colors, etc.
+		if (	aStages[1].rgbGen == CGEN_LIGHTING_DIFFUSE ||
+				aStages[1].rgbGen == CGEN_WAVEFORM ||
+				aStages[1].rgbGen == CGEN_ENTITY ||
+				aStages[1].rgbGen == CGEN_ONE_MINUS_ENTITY ||
+				aStages[1].alphaGen == AGEN_WAVEFORM ||
+				aStages[1].alphaGen == AGEN_LIGHTING_SPECULAR ||
+				aStages[1].alphaGen == AGEN_ENTITY ||
+				aStages[1].alphaGen == AGEN_ONE_MINUS_ENTITY ||
+				aStages[1].alphaGen == AGEN_PORTAL )
+			CollapseFailure;
+
+		// For the remaining cases, we generate and test the colors.
 		static stageVars_t svarsMT;
 		R_ComputeColors( &aStages[1], svarsMT );
 		const int* colors = (const int*)svarsMT.colors;