Add TODO about vertex color linearization to GLSL code

It's not really clear at this moment how we can detect cases
when attribute needs linearization. For now added a comment
so we don't forget about this, hopefully.

source/blender/gpu/shaders/gpu_shader_material.glsl

@@ -2614,6 +2614,9 @@ void node_gamma(vec4 col, float gamma, out vec4 outcol)
 
 void node_attribute(vec3 attr, out vec4 outcol, out vec3 outvec, out float outf)
 {
+	/* TODO(sergey): This needs linearization for vertex color.
+	 * But how to detect cases when input is linear and when it's srgb?
+	 */
 	outcol = vec4(attr, 1.0);
 	outvec = attr;
 	outf = (attr.x + attr.y + attr.z)/3.0;