Since a buffer can be used by multiple surfaces at once,
we need to release the buffer only after all surfaces
are finished with it. Currently we track whether or
not to release the buffer based on the accessible boolean.
This commit changes it to a counter to accomodate multiple
users.
Also, each surface needs to know whether not it is done with
the buffer, so this commit adds a buffer_used boolean to the
surface state.
https://bugzilla.gnome.org/show_bug.cgi?id=761613
bed82427c6