egl/wayland: Ensure we get a back buffer

Commit 9ca6711faa03 changed the Wayland winsys to only block for the
frame callback inside SwapBuffers, rather than get_back_bo. get_back_bo
would perform a single non-blocking Wayland event dispatch, to try to
find any release events which we had pulled off the wire but not
actually processed. The blocking dispatch was moved to SwapBuffers.

This removed a guarantee that we would've processed all events inside
get_back_bo(), and introduced a failure whereby the server could've sent
a buffer release event, but we wouldn't have read it. In clients
unconstrained by SwapInterval (rendering ~as fast as possible), which
were being displayed directly without composition (buffer release delayed),
this could lead to get_back_bo() failing because there were no free
buffers available to it.

The drawing rightly failed, but this was papered over because of the
path in eglSwapBuffers() which attempts to guarantee a BO, in order to
support calling SwapBuffers twice in a row with no rendering actually
having been performed.

Since eglSwapBuffers will perform a blocking dispatch of Wayland
events, a buffer release would have arrived by that point, and we
could then choose a buffer to post to the server. The effect was that
frames were displayed out-of-order, since we grabbed a frame with random
past content to display to the compositor.

Ideally get_back_bo() failing should store a failure flag inside the
surface and cause the next SwapBuffers to fail, but for the meantime,
restore the correct behaviour such that get_back_bo() no longer fails.

Signed-off-by: Daniel Stone <[email protected]>
Reported-by: Eero Tamminen <[email protected]>
Acked-by: Pekka Paalanen <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98833
Fixes: 9ca6711faa03 ("Revert "wayland: Block for the frame callback in get_back_bo not dri2_swap_buffers"")

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