Add support for JPEG output in startmovie

Implement an option to select between TGA and JPEG output for the Source Recorder command `startmovie`. Feature request by [Quadstrike](http://steamcommunity.com/id/Quadstrike/).

* New settings to support this
* Proper UI controls

Details about the command:
```
] startmovie
startmovie <filename>
 [
 (default = TGAs + .wav file)
 h264 = H.264-encoded audio and video (must have QuickTime installed!)
 raw = TGAs + .wav file, same as default
 tga = TGAs
 jpg/jpeg = JPegs
 wav = Write .wav audio file
 jpeg_quality nnn = set jpeq quality to nnn (range 1 to 100), default 50
 ]
examples:
   startmovie testmovie jpg wav jpeg_quality 75
   startmovie testmovie h264   <--- requires QuickTime
AVI is no longer supported.
```