proper solution for variable-args jack_client_open()
skip meter re-draw when no pixels are changed.
combine short periodic timeout functions for windows.

Fixes excessive CPU usage (spinlock instead of sleep/WaitForSingleObject)

MS Windows has a scheduler time-slice of 15ms.
Ardour has two fast timeout functions: The FPS timeout (20-60Hz, clock
video-monitor) and a fast-screen-update (25Hz, meters etc).
They are loosely coupled (async) and scheduled from the main application
thread (gtk main).

Since they're async, gtk schedules them as needed. Even though the actual
period of the separate timeouts is larger. The effective period between
all timeout callbacks is <15ms and the main application thread never idles.