Add temporally overlapping subtitle support.

Add temporally overlapping subtitle support.
* New subtitle sync algorithm added to sync work-object ("simultaneous").
  Classic algorithm preserved but disabled.
* Render work-object now supports queueing a /list/ of subtitles.
* FIFOs have been extended to support pushing/popping buffer-lists as single elements.
* Added SUBSYNC_VERBOSE_TIMING flag to debug timing issues related to subtitle display.

Observable behaviors changed in the new subtitle sync algorithm:
* Temporally overlapping subtitles are no longer trimmed to be non-overlapping.
* Subtitles less than two seconds long are no longer artificially extended. Sorry, Indochine fans.
* Subtitles that stop before they start will never be displayed. The old algorithm will display such subtitles if they begin in the future (relative to the current video frame being processed).

git-svn-id: svn://localhost/HandBrake/trunk@3804 b64f7644-9d1e-0410-96f1-a4d463321fa5