Extend Gawain to use Blender's built-in shaders

Was already done for immediate mode, but rearranged code to make a clean separation. Cleaned up #includes for code that uses this feature.

Added same for batched rendering.

source/blender/gpu/GPU_immediate.h

@@ -31,3 +31,8 @@
 #pragma once
 
 #include "gawain/immediate.h"
+#include "GPU_shader.h"
+
+/* Extend immBindProgram to use Blender’s library of built-in shader programs.
+ * Use immUnbindProgram() when done. */
+void immBindBuiltinProgram(GPUBuiltinShader);