background: Rename visible_region to clip_region

This does better reflect what this region is used for.

https://bugzilla.gnome.org/show_bug.cgi?id=703332
meta-shaped-texture: Don't queue redraws for obscured regions

When we get a damage event we update the window by calling
meta_shaped_texture_update_area which queues a redraw on the actor.
We can avoid that for obscured regions by comparing the damage area to
our visible area.

This patch causes _NET_WM_FRAME_DRAWN messages to be not sent in some cases
where they should be sent; they will be added back in a later commit.

https://bugzilla.gnome.org/show_bug.cgi?id=703332
meta-window-actor: Throttle obscured frame synced apps

We must send frame_drawn and frame_timing messages to even when
we don't actually queue a redraw on screen to comply with the
WM sync spec.

So throttle such apps to down to a ~100ms interval.

https://bugzilla.gnome.org/show_bug.cgi?id=703332