video-timer: add simple example of PLL-style clock adjustment

video-timer: add simple example of PLL-style clock adjustment

The first version of the video-timer simply played back the video
according to the wall clock, and showed each frame at the neareste
presentatin time. But an alternative strategy for playing back
video is that if the frame-rate is an integer-divisor of the
display refresh rate, or very close to that, is to change the playback
speed to complete avoid frame drops and changes in latency.
(This would require resampling audio if present.)

Demonstrate this technique by adding a --pll option to the
video-timer demo.

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