Revert "wayland: Block for the frame callback in get_back_bo not dri2_swap_buffers"

This reverts commit 25cc889004aad6d1cab9edd76db898658e347b97, though
since the code has changed, it was applied manually.

The intent of moving blocking from SwapBuffers to get_back_bo, was to
avoid unnecessary triple-buffering by ensuring that the compositor had
fully processed the previous frame before we started rendering. This
means that the only time we would have to resort to triple-buffering
would be when the buffer is directly scanned out, thus saving an extra
buffer for composition anyway.

The 'repaint window' changes introduced in Weston since then, however,
have narrowed the window of time between the frame event being sent and
the repaint loop needing to conclude, to 7ms by default, in order to
reduce latency. This means however that blocking in get_back_bo gives a
maximum of 7ms for the entire GL submission to begin and complete.

Not only this, but if a client is using buffer_age to avoid full
repaints, the buffer-age request will stall in get_back_bo until the
frame callback completes, meaning that the client cannot even calculate
the repaint area before the 7ms window.

The combination of the two meant that WebKit-GTK+ was failing to
achieve full framerate on a Minnowboard, due to spending a great deal of
its time attempting to query the age of the next buffer before redraw.

Revert to the previous behaviour of allowing rendering to begin but
delaying SwapBuffers, unless and until we can find a more gentle
behaviour.

Signed-off-by: Daniel Stone <[email protected]>
Reviewed-by: Jonas Ådahl <[email protected]>
Reviewed-by: Derek Foreman <[email protected]>
Tested-by: Derek Foreman <[email protected]>
Cc: Kristian Høgsberg <[email protected]>

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