egl/wayland: wait for compositor to release shm buffers

egl/wayland: wait for compositor to release shm buffers

Some Wayland compositors, notably Exo, do not always release buffers
fast enough, and not in sync with their frame callbacks, to guarantee
that a free buffer is available the next time a client calls
`eglSwapBuffers()`.

This currently leads to a crash in `dri2_wl_swrast_get_backbuffer_data()`
with the swrast backend. To avoid this, simply block until the
compositor releases a buffer eventually.

While arguably compositors should release buffers they don't need any
more for the next frame, this can be quite complex depending on
the architecture - notably multi-process/IPC in case of Exo.

cc: mesa-stable

Signed-off-by: Robert Mader <robert.mader@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24091>