Currently, the EGLstream backend would increment the pixmap refcount for
each commit, and decrease that refcount on the wl_buffer release
callback.

But that's relying on the compositor sending us a release callback for
each commit, otherwise the pixmap refcount will keep increasing and the
pixmap will be leaked.

So instead, increment the refcount on the pixmap only when we have not
received a release notification for the wl_buffer, to avoid increasing
the pixmap refcount more than once without a corresponding release
event.

This way, if the pixmap is still in use when released on the X11 side,
the EGL stream will be kept until the compositor releases it.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Suggested-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>