frameclock: Run paint idle from flush idle

Don't return to the main loop, instead force a run of the paint idle.
The paint idle will know to skip all the phases that aren't requested.

This is critically important becuase gdksurface.c assumes the
FLUSH_EVENTS and RESUME_EVENTS phases are matched, and we cannot
guarantee that if we return to the main loop and let various reentrant
code change the frame clock state.

This would lead to bugs with events being paused and never unpaused
again or even crashes.

Fixes #4941

gdk/gdkframeclockidle.c

@@ -373,6 +373,9 @@ gdk_frame_clock_flush_idle (void *data)
   else
     priv->phase = GDK_FRAME_CLOCK_PHASE_NONE;
 
+  g_clear_handle_id (&priv->paint_idle_id, g_source_remove);
+  gdk_frame_clock_paint_idle (data);
+
   return FALSE;
 }