VLC Player fixes and optimizations

Some fixes and optimizations in VLC Player.
Fix FFmpeg Thumbnal AR

Fix aspect ratio of thumbnails generated by FFmpeg, just like MPlayer
does (fitting the thumb in a 320x180 rectangle with black padding).
Fix FFmpeg subtitle convert

Added a stopProcess() at the end of FFmpeg subtitle SRT to ASS
converting process. Without this, a pipe is created and a timer keeps
messing up the buffer progress bar, showing 0 MB/XX MB intermittently.
We can see this happening both in GUI interface and trace log when
playing a video with on-the-fly conversion of SRT subtitle to ASS with
FFmpeg.
Merge pull request #291 from tdcosta100/fix-ffmpeg-subtitle-convert

Fix FFmpeg subtitle convert
Merge pull request #290 from tdcosta100/ffmpeg-thumbnail-ar-fix

Fix FFmpeg Thumbnail AR
Merge branch 'master' of github.com:UniversalMediaServer/UniversalMediaServer into 4.0.0
Merge pull request #280 from tdcosta100/vlc-fixes-optimizations

VLC Player fixes and optimizations
Go back to using MP2 audio in 3.6.x
Merge branch 'master' of github.com:UniversalMediaServer/UniversalMediaServer into 4.0.0
Formatting
Try outputting AC-3 audio instead of MP2 via VLC when transcoding. Needs testing.

In version 2.0.5 of VLC, the following bugs were present when using a52 to convert audio to AC-3:
 - Incorrect channel mapping when input was DTS (from memory, the front-left channel would be heard through the surround-left speaker). This is reportedly fixed in 2.2.0 of VLC.
 - Audio stops transcoding before the end of the video - it happened between 5-45 minutes. Not sure if that's fixed now.

If either of these bugs are still present in VLC 2.2.0+, we should go back to using MP2