renderer: move lightFactor to the end of u_ColorModulateColorGen

Move lightFactor to the end of u_ColorModulateColorGen.

It makes it more future proof by allowing future patches using
new bits to be kept small and not have to care about it.

Author: illwieckz

src/engine/renderer/gl_shader.h

@@ -3607,7 +3607,7 @@ class u_ColorModulateColorGen :
 					// vertexOverbright is only needed for non-lightmapped cases. When there is a
 					// lightmap, this is done by multiplying with the overbright-scaled white image
 					colorModulate |= Util::ordinal( ColorModulate::COLOR_LIGHTFACTOR );
-					lightFactor = uint32_t( tr.mapLightFactor ) << 6;
+					lightFactor = uint32_t( tr.mapLightFactor ) << 26;
 				} else {
 					colorModulate |= Util::ordinal( ColorModulate::COLOR_ONE );
 				}
@@ -3624,10 +3624,10 @@ class u_ColorModulateColorGen :
 
 		if ( useMapLightFactor ) {
 			ASSERT_EQ( vertexOverbright, false );
-			lightFactor = uint32_t( tr.mapLightFactor ) << 6;
+			lightFactor = uint32_t( tr.mapLightFactor ) << 26;
 		}
 
-		colorModulate |= lightFactor ? lightFactor : 1 << 6;
+		colorModulate |= lightFactor ? lightFactor : 1 << 26;
 
 		switch ( alphaGen ) {
 			case alphaGen_t::AGEN_VERTEX:

src/engine/renderer/glsl_source/common.glsl

@@ -77,8 +77,10 @@ Bit 2: color += lightFactor
 Bit 3: alpha * 1
 Bit 4: alpha * ( -1 )
 Bit 5: alpha = 1
-Bit 6-9: lightFactor
-All bits after lightFactor should be left zeroed. */
+Bits 6-25: available for future usage
+Bits 26-30: lightFactor:
+Bit 31: intentionally left blank, some rewrite is required (see below)
+if that bit has to be used once all the other bits are exhausted. */
 
 float colorModArray[3] = float[3] ( 0.0f, 1.0f, -1.0f );
 
@@ -122,9 +124,16 @@ vec4 ColorModulateToColor( const in int colorMod, const in float lightFactor ) {
 
 float ColorModulateToLightFactor( const in int colorMod ) {
 #if defined(HAVE_EXT_gpu_shader4)
-	return float( ( colorMod >> 6 ) & 0xF );
+	/* The day the 31st bit is used, this should be done instead:
+	return float( ( colorMod >> 26 ) & 0xF ); */
+	return float( colorMod >> 26 );
 #else
-	return float( colorMod / 64 );
+	/* The day the 31st bit used, this should be rewritten to
+	extract the value without the sign, like that:
+	int v = colorMod;
+	if ( v < 0 ) v = 2147483647 - abs( v ) + 1;
+	return float( v / 67108864 ); */
+	return float( colorMod / 67108864 );
 #endif
 }