build: require recent libass

Nobody should use an older version. It's perfectly backwards and forward
compatible, so distros have no excuse not to package a recent version.
Older versions lack tons of bug fixes (some of them crashing bugs, and
potentially security relevant).

With love to Debian, which is still on 0.10.2.

README.md

@@ -48,7 +48,7 @@ Essential dependencies (incomplete list):
   At least FFmpeg 2.1.4 or Libav 10 is required.
 - zlib
 - iconv (normally provided by the system libc)
-- libass
+- libass (OSD, OSC, text subtitles)
 - Lua (optional, required for the OSC pseudo-GUI and youtube-dl integration)
 - libjpeg (optional, used for screenshots only)
 - Enca (optional, for subtitle charset detection)

wscript

@@ -272,7 +272,7 @@ iconv support use --disable-iconv.",
     }, {
         'name': '--libass',
         'desc': 'SSA/ASS support',
-        'func': check_pkg_config('libass'),
+        'func': check_pkg_config('libass', '>= 0.12.1'),
         'req': True,
         'fmsg': "Unable to find development files for libass. Aborting. \
 If you really mean to compile without libass support use --disable-libass."