Add contexts to font faces. This avoids font collisions from embedded fonts in video files

Fixes #841
Fix duplicate entries of external subs when in continuous playback mode

Fixes #999
Retry login if opensubtitles query fails with a server side error

Fixes #970
dvd: Fix resource leak
dvd: Fix problems with navigation in some DVD menus

The problem was related to timestamp generation
osx: rm files before cp -a them (symlinks cannot be overwritten by cp)
Decode embedded vobsub (dvbsub) using Showtime's own DVDSPU decoder

Fixes #972
Support external SSA / ASS subtitles

Fixes #590
Redesign the ext sub handling a bit
Fix incorrect handling of alpha values in SSA

Fixes #983
Drop debug printfs
Fix various SSA styling issues
Drop debug printouts
Move dvd language codes to misc/string.c
vda: Avoid spurious frame durations
Add htsmsg_json_serialize_to_rstr()
Add support for vobsub (.idx / .sub) files

Fixes #971
vobsub: Fix issues when seeking
vobsub: Read from sub file on a separate thread to avoid stalling the video thread
vda: Make sure we show video bitrate