gsk: Add gsk_renderer_new_for_window()

gsk: Add gsk_renderer_new_for_window()

and remove gsk_renderer_get_for_display().

This new function returns a realized renderer. Because of that, GSK can
catch failures to realize, destroy the renderer and try another one.

Or in short: I can finally use GTK on Weston with the nvidia binary
drivers again.

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>