All backends follow the same pattern of queueing events first in
ClutterMainContext, then copying them to a ClutterStage queue and
immediately free them. Instead, we can just pass ownership of events
directly to ClutterStage thus avoiding the allocation and copy in
between.
https://bugzilla.gnome.org/show_bug.cgi?id=711857
Acquiring the Clutter lock to mark critical sections is not portable,
and not recommended to implement threaded applications with Clutter.
The recommended pattern is to use worker threads, and schedule UI
updates inside idle or timeout handlers within the main loop. We should
enforce this pattern by deprecating the threads_enter()/leave()
functions. For compatibility concerns, we need internal API to acquire
the main lock during frame processing dispatch.
https://bugzilla.gnome.org/show_bug.cgi?id=679450
