glvideomixer: fix race retrieving the GL context from the display

glvideomixer: fix race retrieving the GL context from the display

_get_gl_context() can be called concurrently from either propose_allocation() or
decide_allocation().  If it so happens that this happens at the same time,
the check for whether we already had a GL context was outside the lock.  Inside
the lock and loop, the first thing that happens is that we unref the current GL
context (if valid) as if there was a conflict adding it to the display.  If the
timing was unlucky, subsequent use of the GL context would be referencing an
already unreffed GL context object resulting in a critical:

g_object_ref: assertion 'object->ref_count > 0' failed

https://bugzilla.gnome.org/show_bug.cgi?id=766703