PMP will now download codec binaries from nightlies on playback start if
built with -DENABLE_CODECS=on. With "off" (the default), most codecs on
demand code will be built, but the list of downloadable codecs will be
empty.
This uses a hardcoded CODECS_BUILD_NUMBER. This is sort of ok, as long
as the list of codecs/names on nightlies doesn't change. If there's a
change, it should be updated manually. (Same if new platforms are added
etc.) Note that even if this is not changed, the code will download the
correct codecs by looking at the FFmpeg version number libmpv is linked
against.
