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/shaders/gpu_shader_vertex.glsl

@@ -14,6 +14,58 @@ varying vec3 varnormal;
 varying float gl_ClipDistance[6];
 #endif
 
+float srgb_to_linearrgb(float c)
+{
+	if (c < 0.04045)
+		return (c < 0.0) ? 0.0 : c * (1.0 / 12.92);
+	else
+		return pow((c + 0.055) * (1.0 / 1.055), 2.4);
+}
+
+void srgb_to_linearrgb(vec3 col_from, out vec3 col_to)
+{
+	col_to.r = srgb_to_linearrgb(col_from.r);
+	col_to.g = srgb_to_linearrgb(col_from.g);
+	col_to.b = srgb_to_linearrgb(col_from.b);
+}
+
+void srgb_to_linearrgb(vec4 col_from, out vec4 col_to)
+{
+	col_to.r = srgb_to_linearrgb(col_from.r);
+	col_to.g = srgb_to_linearrgb(col_from.g);
+	col_to.b = srgb_to_linearrgb(col_from.b);
+	col_to.a = col_from.a;
+}
+
+bool is_srgb(int info)
+{
+#ifdef USE_NEW_SHADING
+	return (info == 1)? true: false;
+#else
+	return false;
+#endif
+}
+
+void set_var_from_attr(vec3 attr, int info, out vec3 var)
+{
+	if (is_srgb(info)) {
+		srgb_to_linearrgb(attr, var);
+	}
+	else {
+		var = attr;
+	}
+}
+
+void set_var_from_attr(vec4 attr, int info, out vec4 var)
+{
+	if (is_srgb(info)) {
+		srgb_to_linearrgb(attr, var);
+	}
+	else {
+		var = attr;
+	}
+}
+
 void main()
 {
 #ifndef USE_OPENSUBDIV