Get rid of the strange and messy reliance on DEMUXER_TYPE_ constants.
Instead of having two open functions for the demuxer callbacks (which
somehow are both optional, but you can also decide to implement both...),
just have one function. This function takes a parameter that tells the
demuxer how strictly it should check for the file headers. This is a
nice simplification and allows more flexibility.
Remove the file extension code. This literally did nothing (anymore).
Change demux_lavf so that we check our other builtin demuxers first
before libavformat tries to guess by file extension.
Old code used to use libass' recoding feature, which is a copy of the
old MPlayer code. We dropped that a few commits ago. Unfortunately,
this made it impossible to load some subtitle files, like UTF-16 files.
Make .ass loading respect -subcp again. We do this by recoding the
probe buffer to UTF-8, and then trying to load it normally. (Yep.)
Since UTF-16 in particular will effectively half the probe buffer size,
double the probe size.
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.
