backends/wayland: Avoid rendering to buffers held by the compositor

Currently, the wayland backend does not monitor when its buffers are
released by the host compositor, which can potentially result in
glitches.

This change refactors the wayland backend so the swapchain buffers are
released upon receiving wl_buffer.release event.

wl_buffer.release event handling lives in the WaylandBackend. This
allows us to share some code, it might also be useful for implementing
direct scanout as any GraphicsBuffer providing dmabuf or shm attributes
can be wrapped in a wl_buffer.