Add buffering to video renderers (without dropping)

On request by elupus this is #2249 without lateness detection and dropping control in dvdplayer. It's more convenient for review but I don't recommend to test this pr alone because the old lateness detection won't work well with buffering.

@bobo1on1
Do you have a good idea how to calculate absolute timestamps in advance without breaking "sync playback to display"? In this pr I have moved this code from dvdplayer to rendermanager. This way it works the same way as it did before but it requires RenderManager to have access to dvdclock.