Properly handle vertex color color space for Cycles GLSL

A bit tricky, need to pass additional information about what the attribute
is and how to deal with it.

BI path stays unchanged, just to make things simplier for now.

Fixes T48555: Cycles GLSL- Incorrect Vertex Color results from Attribute node

source/blender/gpu/GPU_shader.h

@@ -104,6 +104,7 @@ typedef struct GPUVertexAttribs {
 	struct {
 		int type;
 		int glindex;
+		int glinfoindoex;
 		int gltexco;
 		int attribid;
 		char name[64];	/* MAX_CUSTOMDATA_LAYER_NAME */