Send _NET_WM_FRAME_TIMINGS messages

We previously had timestamp information stubbed out in
_NET_WM_FRAME_DRAWN. Instead of this, add a high-resolution timestamp
in _NET_WM_FRAME_DRAWN then send a _NET_WM_FRAME_TIMINGS message
after when we have complete frame timing information, representing
the "presentation time" of the frame as an offset from the timestamp
in _NET_WM_FRAME_DRAWN.

To provide maximum space in the messages,_NET_WM_FRAME_DRAWN and
_NET_WM_FRAME_TIMINGS are not done as WM_PROTOCOLS messages but
have their own message types.

https://bugzilla.gnome.org/show_bug.cgi?id=685463