sub: add demux_libass wrapper, drop old hacks

demux_libass.c allows us to make subtitle format detection part of the
normal file loading process. libass has no probe function, but trying to
load the start of a file (the first 4 KB) is good enough. Hope that
libass can even handle random binary input gracefully without printing
stupid log messages, and that the libass parser doesn't accept too many
non-ASS files as input.

This doesn't handle the -subcp option correctly yet. This will be fixed
later.

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