GPUShader: Add selection id shader

This is to separate id drawing from standard color drawing.

source/blender/gpu/shaders/gpu_shader_selection_id_frag.glsl

@@ -0,0 +1,21 @@
+
+#ifdef UNIFORM_ID
+uniform uint id;
+#else
+flat in vec4 id;
+#endif
+
+out vec4 fragColor;
+
+void main()
+{
+#ifdef UNIFORM_ID
+	fragColor = vec4(
+		((id      ) & uint(0xFF)) * (1.0f / 255.0f),
+		((id >>  8) & uint(0xFF)) * (1.0f / 255.0f),
+		((id >> 16) & uint(0xFF)) * (1.0f / 255.0f),
+		((id >> 24)             ) * (1.0f / 255.0f));
+#else
+	fragColor = id;
+#endif
+}