[mythtv] vsync and frame timing adjustments

[Daniel Thor Kristjansson]

While you guys are talking about this, I'd like to suggest that
PrepareFrame() should be called earlier, maybe on the previous
frame. Ideally how many frames ahead it's called could be adjusted.
With HDTV and a GeForce FX 5200 using XvMC it seems to take about 40ms
from the time you call PrepareFrame() before it's ready to be shown,
which is a real problem if you are aiming for a frame rate of 30fps (or
33ms between frames) because we call it at most 33 ms before calling
Show().

-- Daniel

PS  40ms is an example, it's sometimes less, sometimes more. It takes
the least time when there is no OSD, and the longest when the OSD is
partially transparent.