clients/constraints: Fix up buffer handling
Clear the selected buffer pointer immediately before the array walk to pick a new buffer so we don't accidentally re-use the attached buffer when it's already in use.
Set the buffer used bit only when we attach and commit a buffer - this way we don't accidentally consume all our buffers with no way to have them released.
Clean up buffers based on wl_buffer presence instead of used at end of run.
Signed-off-by: Derek Foreman derek.foreman@collabora.com