Made Bitmap::Save() and Dump() const.
Use Bitmap::Save() instead of the SDL methods.
Verified and removed TODO comment
Cleanups. Corrected wrong cast.\
Introduce Bitmap::Load() versions which load from a file and from a
block of memory. USe the latter in BMPResource instead of using SDL
methods.
Made Bitmap::Surface() private, and also Bitmap constructor.
Change some parameters type.
Reordered methods in header.
Moved include "SDL-h" to Bitmap.cpp.