Added current_w and current_h to the SDL_VideoInfo structure, which is set to the desktop resolution during video intialization, and then set to the current resolution when a video mode is set.

SDL_SetVideoMode() now accepts 0 for width or height and will use the current video mode (or the desktop mode if no mode has been set.)

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401549