window: implement per-surface redraws

window: implement per-surface redraws

Add redraw_needed flag to all surfaces, in addition to having one in
window. The window redraw_needed flag is changed to force a redraw of
the whole window, regardless of frame events.

widget_schedule_redraw() now schedules the redraw only for the surface,
where the widget is on. window_schedule_redraw() is equivalent to
scheduling a redraw for all (sub-)surfaces of the window.

We still use only one deferred task for all redraws.

surface_redraw() will skip the redraw, if the window does not force a
redraw and the surface does not need a redraw. It will also skip the
redraw, if the frame callback from the previous redraw has not triggered
yet. When the frame callback later arrives, the redraw task will be
scheduled, if the surface still needs a redraw.

If the window forces a redraw, the redraw is executed even if there is a
pending frame callback. This is for resizing: resizing should trigger a
window repaint, as it really wants to update all surfaces in one go, to
apply possible sub-surface size and position changes. Resizing is the
only thing that makes a window force a redraw.

With this change, subsurfaces demo can avoid repainting the cairo
sub-surface while still animating the GL sub-surface.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>