RFC: GRAPHICS: Implement JPEGDecoder based on libjpeg.

To make life a bit easier for everyone in the recent JPEG discussion (started with pull request #374) I hacked up some JPEGDecoder implementation based on libjpeg. I only tested it with libjpeg-turbo (that's shipped with my Linux distribution). For testing I have used the demo of Carol Reed 6: Black Circle (http://www.mdna-games.com/Black_Circle/demo.htm). That seems to work fine (well apart it from erroring out on me at some point inside the Wintermute code because it uses some absolute paths). Otherwise this is completely untested (especially the Groovie adaptions).

Obvious downside: Groovie now depends on the external dependency libjpeg.

Hopefully, this is of use for everybody who wants to make his mind clear on the various possible ways we can go for better JPEG support.