Dropped the direct jpeg dependency.

This is resolving #207.

- I removed the specific GetImageInfo() in favour of something
  using SDL_Image.
- I dropped writing a JPG screenshot in favour of a lossless png one.
  and I fixed the png writing function which was causing a crash
  when handling a rgb formatted image.

As a result, the engine can now load images from every formats
SDL_Image can support and will create nicer screenshots.