render/allocator/gbm: implement begin_data_ptr_access

Allows CPU access of the buffer. Can be useful for multi-GPU copies
via CPU.

Might be quite slow, and might fail for an arbitrary driver-specific
reason, so not advertised in wlr_allocator.buffer_caps.

include/render/allocator/gbm.h

@@ -12,6 +12,7 @@ struct wlr_gbm_buffer {
 	struct wl_list link; // wlr_gbm_allocator.buffers
 
 	struct gbm_bo *gbm_bo; // NULL if the gbm_device has been destroyed
+	void *gbm_map_data; // NULL unless we have an active mapping
 	struct wlr_dmabuf_attributes dmabuf;
 };