And use it to store the client pointer instead of storing it in the buffer data.
My main motivation is to make it easy to implement alternative solutions, such as `mach_vm_remap` or `glImportMemoryFdEXT`. However, I think this change is worth it on its own as the current strategy violates the spec by modifying pointers returned from read-only mappings. It will also make it possible to fix wow64 buffer leaks.