When we were shaping the window with a cairo region, there was an easy
optimization to restrict painting only to the pixels we were going to
actually draw. With rounded corners, the amount of work we have to do
figure out what pixels isn't worth the small savings of not drawing the
completely transparent parts of the corners, so remove this optimization,
and the supporting meta_shaped_texture_get_visible_pixels_region()

https://bugzilla.gnome.org/show_bug.cgi?id=657639