sub: redo how -no-ass is handled

The -no-ass switch used to disable any use of libass for text subtitles.
This is not really the case anymore, because libass is now always
involved when rendering text. The only remaining use of -no-ass is
disabling styling or showing subtitles on the terminal. On the other
hand, the old subtitle rendering path is a big reason why the subtitle
code is still a big mess with an awful number of obscure special cases.

In order to simplify it, remove the old subtitle rendering code, and
always go through sd_ass.c. Basically, we use ASS_Track as central data
structure for storing text subtitles instead of struct sub_data. This
also makes libass mandatory for all text subs, even if they are printed
to the terminal in -no-video mode. (We could add something like sd_text
to avoid this, but it's not worth the trouble.)

struct sub_data and subreader.c are still around, even its ASS/SSA
reader. But struct sub_data is freed right after converting it to
ASS_Track. The internal ASS reader actually can handle some obscure
cases libass can't, like files encoded in UTF-16.
sub: remove check_duplicate_plaintext_event()

This was once needed to handle subtitle packages coming from a demuxer,
where seeking back might repeat previous events. This doesn't happen
anymore, and this code is used to convert complete files. So if there
are any duplicate lines, they must have been duplicated in the file,
and the old subtitle renderer would have shown them twice as well.

Today checking for duplicate events happens in sd_ass.c (and has been
for a while). There's no reason to keep this code, and it actually
causes trouble. Loading big subtitle files is extremely slow because
this makes adding n subtitles O(n^2).
sub: use DVD PTS fallback code in normal sub decoding path

It appears demux_mpg doesn't output timestamps for subtitles. The vobsub
code handled this by doing its own PTS calculations. This code is absent
from the normal subtitle decoder path. Copy this code into the normal
path, so that we can unify the subtitle decoder paths in a later commit.

Decoding subtitles with sd_lavc when playing DVD with demux_mpg still
doesn't work.
sub: add sd_spu.c to wrap spudec, cleanup mplayer.c

This unifies the subtitle rendering path. Now all subtitle rendering
goes through sd_ass.c/sd_lavc.c/sd_spu.c.

Before that commit, the spudec.h functions were used directly in
mplayer.c, which introduced many special cases. Add sd_spu.c, which is
just a small wrapper connecting the new subtitle render API with the
dusty old vobsub decoder in spudec.c.

One detail that changes is that we always pass the palette as extra
data, instead of passing the libdvdread palette as pointer to spudec
directly. This is a bit roundabout, but actually makes the code simpler
and more elegant: the difference between DVD and non-DVD dvdsubs is
reduced.

Ideally, we would just delete spudec.c and use libavcodec's DVD sub
decoder. However, DVD playback with demux_mpg produces packets
incompatible to lavc. There are incompatibilities the other way around
as well: packets from libavformat's vobsub demuxer are incompatible to
spudec.c. So we define a new subtitle codec name for demux_mpg subs,
"dvd_subtitle_mpg", which only sd_spu can decode.

There is actually code in spudec.c to "assemble" fragments into complete
packets, but using the whole spudec.c is easier than trying to move this
code into demux_mpg to fix subtitle packets.

As additional complication, Libav 9.x can't decode DVD subs correctly,
so use sd_spu in that case as well.
spudec: restore --sub-forced-only support

This was broken with 84829a4 "Merge branch 'osd_changes' into master".
The new OSD/subtitle code never respected the --sub-forced-only option,
and the old code containing the code for this was removed in fd5c4a1.
options: add --no-sub-visibility for symmetry

Not really useful, but for symmetry with the sub-visibility property
(mapped to the 'v' key by default).
sub: remove some global variables
sub: various minor subtitle related changes

Just pushing some code around.
core: add demux_sub pseudo demuxer

Subtitle files are opened in mplayer.c, not using the demuxer
infrastructure in general. Pretend that this is not the case (outside of
the loading code) by opening a pseudo demuxer that does nothing. One
advantage is that the initialization code is now the same, and there's
no confusion about what the difference between track->stream,
track->sh_sub and mpctx->sh_sub is supposed to be.

This is a bit stupid, and it would be much better if there were proper
subtitle demuxers (there are many in recent FFmpeg, but not Libav). So
for now this is just a transition to a more proper architecture. Look
at demux_sub like an artifical limb: it's ugly, but don't hate it - it
helps you to get on with your life.
sub: refactor

Make the sub decoder stuff independent from sh_sub (except for
initialization of course). Sub decoders now access a struct sd only,
instead of getting access to sh_sub. The glue code in dec_sub.c is
similarily independent from osd.

Some simplifications are made. For example, the switch_id stuff is
unneeded: the frontend code just has to make sure to call osd_changed()
any time subtitles are switched.

This is also preparation for introducing subtitle converters. It's much
cleaner to completely separate demuxer header/renderer glue/decoders
for this purpose, especially since sub converters might completely
change how demuxer headers have to be interpreted.

Also pass data as demux_packets. Currently, this doesn't help much, but
libavcodec converters might need scary stuff like packet side data, so
it's perhaps better to go with passing packets.
sub: basic subtitle converters

Add a basic infrastructure for subtitle converters. These converters
work sort-of like decoders, except that they produce packets instead
of subtitle bitmaps. They are put in front of actual decoders.

Start with sd_movtext. 4 lines of code are blown up to a 55 lines file,
but fortunately this is not going to be that bad for the following
converters.
sub: split subassconvert.c into sd_microdvd.c and sd_srt.c
sub: turn subassconvert_ functions into sub converters

This means subassconvert.c is split in sd_srt.c and sd_microdvd.c. Now
this code is involved in the sub conversion chain like sd_movtext is.
The invocation of the converter in sd_ass.c is removed.

This requires some other changes to make the new sub converter code work
with loading external subtitles. Until now, subtitles loaded via
subreader.c was assumed to be in plaintext, or for some formats, in ASS
(except in -no-ass mode). Then these were added to an ASS_Track. Change
this so that subtitles are always in their original format (as far as
decoders/converters for them are available), and turn every sub event
read by subreader.c as packet to the dec_sub.c subtitle chain.

This removes differences between external/demuxed and -ass/-no-ass code
paths further.
sub: make use of libavcodec subtitle converters

This allows using some formats that were not supported until now, like
WebVTT.

We still prefer the internal subtitle reader (subreader.c), because
1. Libav, and 2. random things which we probably want to keep, such as
control over formatting, codepage stuff, or various mysterious
postprecessing done in that code.
demux_lavf: always set packet duration

Makes WebVTT actually work.