tr_shader: merge legacy lightmap stage into diffusestage

- avoid double shadowing
- avoid to paint shadow over glowmap

Author: illwieckz

src/engine/renderer/tr_shader.cpp

@@ -3938,6 +3938,7 @@ static void CollapseStages()
 	int specularStage = -1;
 	int materialStage = -1;
 	int reflectionStage = -1;
+	int lightStage = -1;
 	int glowStage = -1;
 
 	for ( int i = 0; i < MAX_SHADER_STAGES; i++ )
@@ -3962,6 +3963,10 @@ static void CollapseStages()
 		{
 			reflectionStage = i;
 		}
+		else if ( stages[ i ].type == stageType_t::ST_LIGHTMAP )
+		{
+			lightStage = i;
+		}
 		else if ( stages[ i ].type == stageType_t::ST_GLOWMAP )
 		{
 			glowStage = i;
@@ -4038,6 +4043,14 @@ static void CollapseStages()
 				// disable since it's merged
 				stages[ materialStage ].active = false;
 			}
+			if ( lightStage != -1 )
+			{
+				// merge with diffuse stage
+				stages[ diffuseStage ].rgbGen = stages[ lightStage ].rgbGen;
+				stages[ diffuseStage ].alphaGen = stages[ lightStage ].alphaGen;
+				// disable since it's merged
+				stages[ lightStage ].active = false;
+			}
 			if ( glowStage != -1 )
 			{
 				// merge with diffuse stage