GdkWindowX11: Only start a frame when we emit damage

Instead of communicating the start of a frame to the window manager
as soon as we begin a frame, start a frame only when we know we've
actually created damage to the contents of a window.

(This uses cairo_set_mime_data() as a notification mechanism - a
clever suggestion from Uli Schlachter.)

The advantage of this is that we aren't forcing the compositor to
do a frame cycle and send _NET_WM_FRAME_DRAWN - depending on how the
compositor is structured that might either cause it to do extra
work or it might send _NET_WM_FRAME_DRAWN early and upset frame
timing.

https://bugzilla.gnome.org/show_bug.cgi?id=685460