GPUShader: Add selection id shader

This is to separate id drawing from standard color drawing.

source/blender/gpu/GPU_shader.h

@@ -352,6 +352,9 @@ typedef enum GPUBuiltinShader {
 	GPU_SHADER_2D_UV_EDGES_SMOOTH,
 	GPU_SHADER_2D_UV_FACES,
 	GPU_SHADER_2D_UV_FACES_STRETCH,
+	/* Selection */
+	GPU_SHADER_3D_FLAT_SELECT_ID,
+	GPU_SHADER_3D_UNIFORM_SELECT_ID,
 
 	GPU_NUM_BUILTIN_SHADERS /* (not an actual shader) */
 } GPUBuiltinShader;