Better b-frame muxing. Instead of using an arbitrarily long offset and then later...

Better b-frame muxing. Instead of using an arbitrarily long offset and then later walking the file to find the smallest one, now x264 uses the smallest delay it can from the start. This significantly cuts down muxing time.

As a side effect, it also makes fixing chapter markers + b-frames easy, so that's included too in muxmp4 (thanks, van!).

git-svn-id: svn://localhost/HandBrake/trunk@1152 b64f7644-9d1e-0410-96f1-a4d463321fa5