We need to keep the active buffer (the one we have retreive a
texture id from) otherwise it's racy and upstream may upload
new content before we have rendered or during later redisplay.
This is a "pseudo" base class. Basically it's a shared instance
and class structure and a shared set of function between the
two widget. It cannot have it's own type like normal base class
since the one instance will implement GtkGLArea while the other
implements GtkDrawingAreay. To workaround this, the parent instance
and class is a union of both.
https://bugzilla.gnome.org/show_bug.cgi?id=752441
