Improve GtkWidget redraw queueing in WaylandCompositorDispmanX

Only queue the GtkWidget redraw when a new proxy buffer is created.
This ensures that the widget gets redrawn when there's some insurance
that fresh pixel data will become available through the proxy drivers.
Merge pull request #39 from zdobersek/webkitgtk

Improve GtkWidget redraw queueing in WaylandCompositorDispmanX