LightingShaderGen: Always calculate lighting for both color channels

Cel-damage uses the color from the lighting stage of the vertex pipeline
as texture coordinates, but sets numColorChans to zero.

We now calculate the colors in all cases, but override the color before
writing it from the vertex shader if numColorChans is set to a lower value.

Source/Core/VideoCommon/LightingShaderGen.h

@@ -45,5 +45,5 @@ static const char s_lighting_struct[] = "struct Light {\n"
                                         "};\n";
 
 void GenerateLightingShaderCode(ShaderCode& object, const LightingUidData& uid_data, int components,
-                                u32 numColorChans, const char* inColorName, const char* dest);
+                                const char* inColorName, const char* dest);
 void GetLightingShaderUid(LightingUidData& uid_data);