Introduce wlr_shm_client_buffer, which provides a wlr_buffer wrapper
around wl_shm_buffer.
Because the client can destroy the wl_buffer while we still are using
it, we need to do some libwayland tricks to still be able to continue
accessing its underlying storage. We need to reference the wl_shm_pool
and save the data pointer.
This new API allows buffer implementations to know when a user is
actively accessing the buffer's underlying storage. This is
important for the upcoming client-backed wlr_buffer implementation.
Consumers call wlr_buffer_lock. Once all consumers are done with the
buffer, only the producer should have a reference to the buffer. In this
case, we can release the buffer (and let the producer re-use it).
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.
