gtkpixelcache: Fix possibly ignoring queued redraw

gtkpixelcache: Fix possibly ignoring queued redraw

If a queue_redraw() (invalidating a region, or the whole widget) was
called from the draw() call, it could get ignored if surface_dirty
existed, as it would then be updated, but destroyed right at the end of
the _gtk_pixel_cache_repaint(), leading the next call to
_gtk_pixel_cache_draw() have its call to repaint() be a no-op
(since there's no surface_dirty) and then simply draw from (non
updated) surface.

Signed-off-by: Olivier Brunel <jjk@jjacky.com>
https://bugzilla.gnome.org/show_bug.cgi?id=711545