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.
sd_ass: strip empty/whitespace lines in -no-ass mode

Will just destroy output. In some cases empty newlines might be used by
bad scripts for spacing; too bad for them.
sd_lavc_conv: add hack if AV_CODEC_PROP_TEXT_SUB is not available

Otherwise this could happily open decoders for image subtitles or even
audio/video decoders. AV_CODEC_PROP_TEXT_SUB is a preprocessor symbol,
but it's still better to detect this properly instead of using #ifdef,
because these flags might as well be changed into enums sooner or later.
sub: use libass even if -no-ass is used

The -no-ass option used to disable all use of libass completely. This
doesn't work this way anymore, and the text subtitle path has an
inherent dependency on libass. Currently -no-ass does 3 things:

1. Strip tags and formatting on display, and use a separate renderer for
   the result. (Which might be the terminal, or libass via OSD code.)
2. Not loading attached fonts from Matroska files.
3. Use subreader.c instead of libass for reading .ass files.

1. and 2. are ok and what the user (probably wants), but 3. doesn't
really make sense anymore. subreader.c reads .ass files just fine, but
then does some strange things to them (something about coalescing and
re-adding newlines?), leading to even more broken display with -no-ass.
Instead of fighting with subreader.c, just use libass as loader.
sd_lavc_conv: strip style header

Normally, libavcodec subtitle converters will output a style header like
this as part of the extradata:

    Style: Default,Arial,16,&Hffffff,&Hffffff,&H0,&H0,0,0,0,1,1,0,2,10,10,10,0,0

We don't want that, so use some bruteforce to get rid of them.
sub: don't check for duplicates on sub conversion

This mirrors commit "sub: remove check_duplicate_plaintext_event()".
That code was basically duplicated. In general, this code is still
needed when doing conversion during demuxing (mostly because you can
seek during demuxing, which will cause duplicate events by replaying).
sub: never set VSFilter aspect if the ASS subtitle is converted

When e.g. converting SRT to ASS, we certainly don't want them stretched
by video aspect ratio, even if that's necessary for native ASS
subtitles.

Annoying weird details...
sub: pass subtitle packets as demux_packet

Before this, subtitle packets were returned as data ptr/len pairs, and
mplayer.c got the rest (pts and duration) directly from the demuxer
dasta structures. Then mplayer.c reassembled the packet data structure.

Pass packets directly instead. The mplayer.c side stays a bit awkward,
because the (now by default unused) DVD path keeps getting in the way.
In demux.c there's lots of weird stuff (3 functions that read packets,
really?), but we want to keep the code equivalent for now to avoid
hitting weird issues and corner cases.
core: add common function to initialize AVPacket

Audio and video had their own (very similar) functions to initialize an
AVPacket (ffmpeg's packet struct) from a demux_packet (mplayer's packet
struct). Add a common function for these.

Also use this function for sd_lavc_conv. This is actually a functional
change, as some libavfilter subtitle demuxers add weird out-of-band
stuff as side-data.
sd_ass: make sure user set sub style is used for unstyled subtitles

The default style is added by mp_ass_default_track(), but not by
ass_new_track(). Considering this, the previous condition at this point
didn't make much sense anymore: the actual (converted) subtitle format
doesn't matter much for what styling should be applied. What matters is
if the subtitle was originally ASS, or was converted to it.

In theory it could be possible that the source subtitle is in a format
that like ASS requires full styling to be displayed correctly. But there
isn't any example for this, so we don't consider it.