Split out the client/resource handling out of wlr_buffer by introducing
wlr_client_buffer. Make wlr_buffer an interface so that compositors can
create their own wlr_buffers (e.g. backed by GBM, like glider [1]).
[1]: c66847dd1c/include/gbm_allocator.h (L7)
In case the texture can't be imported, release the buffer so that the
client can submit another one. In case the allocation fails, disconnect
the client.
After some discussions on #wayland, it seems that as soon as you
hold a reference to a DMA-BUF (via EGLImage for instance), the
underlying memory won't get free'd. The client is allowed to
re-use the DMA-BUF and upload something else to it though.
