glx: implement drawable refcounting.

glx: implement drawable refcounting.

The current dri context unbind logic will leak drawables until the process
dies (they will then get released by the GEM code). There are two ways to fix
this: either always call driReleaseDrawables every time we unbind a context
(but that costs us round trips to the X server at getbuffers() time) or
implement proper drawable refcounting. This patch implements the latter.

Signed-off-by: Antoine Labour <piman@chromium.org>
Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>