osd_libass: actually free ASS_Tracks

Not a real leak, just for getting clean valgrind reports on exit.
audio/decode: remove vararg from ad_control()

This was unused and dumb. Ancient MPlayer used varargs instead of void*
arguments for control() functions, and this was the last leftover.
sub: add --osd-blur and --sub-text-blur options

These require bleeding edge libass (latest git version), and will be
ignored otherwise.

I'm not sure about the blur factor and scaling. The ASS/VSFilter
semantics for blur scaling are a bad mess. Might require further
investigation.