GPU: FrameBuffer: Remove GPUFrameBuffer wrapper opaque type

This is the first step into merging DRW_gpu_wrapper.hh into
the GPU module.

This is very similar to #119825.

Pull Request: https://projects.blender.org/blender/blender/pulls/146372

source/blender/python/gpu/gpu_py_state.cc

@@ -500,7 +500,7 @@ static PyObject *pygpu_state_active_framebuffer_get(PyObject * /*self*/)
 {
   BPYGPU_IS_INIT_OR_ERROR_OBJ;
 
-  GPUFrameBuffer *fb = GPU_framebuffer_active_get();
+  blender::gpu::FrameBuffer *fb = GPU_framebuffer_active_get();
   return BPyGPUFrameBuffer_CreatePyObject(fb, true);
 }