Implement SDL_image into RAGE::SDL

[asmodehn]

XorfacX:
Implement SDL_image into RAGE::SDL in order to allow other image format (png, jpg, ...)

[asmodehn]

first 20 percent : localise all the SDL_LoadBMP() calls in the RGBSurface Class ( icon creation must then use it).
then it will be quite straight forward to add more file formats...

[asmodehn]

Implemented the major part of it.
Now the RGBSurface is created with IMG_Load if available.
SDL_image is also detected within the CMake build process.
Just need to see what is actually needed, and test the implementation.
The last bit will be make a small test just to load a png file and display it.

[asmodehn]

XorfacX:
The construtor
RGBSurface::RGBSurface( std::string filename, const Color & colorKey )throw (std::logic_error)
doesn't use IMG_Load and so it need to be corrected ;)