Variable frame rate improvements.

Variable frame rate improvements.
- Makes it work better with the source's time stamps and avoids discontinuities, by removing its lazy assumption that all frames are 3003 ticks long.
- Handles more than 1 frame being dropped out of 5.
- Preserves chapter markers from being dropped along with frames they're attached to (thanks van!).
- Also, since VFR no longer requires input at 29.97 fps, removes the hard-coding of that frame rate.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1249 b64f7644-9d1e-0410-96f1-a4d463321fa5