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>