#53 "ensure A/V synchronization" for subtitles too

[urrameu]

When the "ensure A/V synchronization" option is enabled, extracting subtitles should also use the audio track to maintain correct timing. The mencoder command line should not contain "-nosound", but instead (for example) "-oac copy". This would have virtually no I/O, temp disk space, or CPU overhead but (in my limited tests) solves sync problems with subtitles. It is also (hopefully!) a very simple change to implement.