Add RSF_FORCE_LIGHTMAP

src/engine/renderer/tr_local.h

@@ -1319,6 +1319,8 @@ enum class shaderProfilerRenderSubGroupsMode {
 
 		int		autoSpriteMode;
 
+		bool forceLightMap;
+
 		uint8_t         numDeforms;
 		deformStage_t   deforms[ MAX_SHADER_DEFORMS ];
 

src/engine/renderer/tr_shader.cpp

@@ -5869,6 +5869,14 @@ static shader_t *FinishShader()
 	numStages = MAX_SHADER_STAGES;
 	GroupActiveStages();
 
+	if ( shader.forceLightMap && numStages == 1 ) {
+		shaderStage_t* pStage = &stages[0];
+
+		if ( pStage->type == stageType_t::ST_COLORMAP ) {
+			pStage->type = stageType_t::ST_DIFFUSEMAP;
+		}
+	}
+
 	// set appropriate stage information
 	for ( size_t stage = 0; stage < numStages; stage++ )
 	{
@@ -6233,6 +6241,11 @@ shader_t       *R_FindShader( const char *name, shaderType_t type, int flags )
 		shader.entitySpriteFaceViewDirection = true;
 	}
 
+	if ( flags & RSF_FORCE_LIGHTMAP )
+	{
+		shader.forceLightMap = true;
+	}
+
 	// attempt to define shader from an explicit parameter file
 	shaderText = FindShaderInShaderText( strippedName );
 

src/engine/renderer/tr_types.h

@@ -90,7 +90,7 @@ enum RegisterShaderFlags_t {
 	RSF_NOMIP = BIT( 2 ),
 	RSF_FITSCREEN = BIT( 3 ),
 	RSF_LIGHT_ATTENUATION = BIT( 4 ),
-	RSF_NOLIGHTSCALE = BIT( 5 ), // TODO(0.56): delete, does nothing
+	RSF_FORCE_LIGHTMAP = BIT( 5 ), // Used to make particles/trails work with the lightGrid in GLSL
 	RSF_SPRITE = BIT( 6 ),
 };