Having StDrawingArea use ClutterCairoTexture causes circularity
problems with sizing - StDrawingArea wants to use its allocation for
the size of the texture, but ClutterTexture wants to use the size of
the texture to determine the requited size.
Avoid this by making StDrawingArea directly use Cairo and CoglTexture;
while doing this, the API is changed a bit for simplicity and to
match our use case:
- Instead of clutter_cairo_texture_create(), we have
st_drawing_area_get_context() to retrieve an already created
context. This can only be called in the ::repaint signal.
- The ::redraw signal is changed to ::repaint so we can have
st_drawing_area_queue_repaint() that doesn't collide with
clutter_actor_queue_redraw()
- ::repaint is now emitted lazily when painting the actor rather
than synchronously at various different points.
https://bugzilla.gnome.org/show_bug.cgi?id=611750
It's nicer to have ShellDrawingArea as a St widget so it can
participate more cleanly in CSS styling, such as queuing a redraw
automatically on style changes, and allowing subclasses to use
CSS styling.
https://bugzilla.gnome.org/show_bug.cgi?id=602131
