Changelog:
* MPEG Audio Decoder: Updated liba52
* MPEG Audio Decoder: Updated libdts
* VSFilter: Added RealText support
* VSFilter: Fix rare seeking issue with VobSubs
* VSFilter: Fix potential jitter issues with pos overrides in VSFilter.
* VSFilter: Really fix two subtitle rasterisation bugs: Go back to old pixmix/pixmix2 architecture originally used in Subtitler and early versions of VSFilter, implement an SSE2 version of pixmix2 and ensure (widened-region minus fill) math doesn't underflow. Completely fixes "e1 stray dots" and "vector clip kills alpha" bugs.
Also add several comments explaining the rasterisation code.
* VSFilter: Add support for variable strength and animated e, and implement gaussian lur as an alternative. (lur also does edge blur like e but in a nicer way.)
* VSFilter: Add new override tags xbord ybord xshad yshad to control border and shadow size separately in X and Y directions.
* VSFilter: Add "inverse clip" tag iclip to specify what to hide rather than what to show.
* VSFilter: Partial workaround for problem where border colour shows through fill colour during fad and fade fades. This workaround potentially causes small seams to be visible during the fade, unfortunately this is the price to pay for getting correct fades.