libweston: Fix clear timing of output repainted flag

Since the repaint status of the flushed output may be reset if a output
repaint is failed, it is necessary to clear the repainted flag
immediately after output repaint flush/cancel.

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>