Before this commit loadPNG() could return any surface format and the caller
was responsible for converting it to the desired format. However, in practice
all callers want a surface with an alpha channel and SDL only supports that
with 32bpp surfaces, so RGBA or a permutation thereof. So I changed the
contract for loadPNG() so it is required to return an RGBA surface, and
removed the conversion code in the callers.
The next step is to replace IMG_Load() by a function that calls libpng
directly and loads a fixed 32bpp pixel format. That way, we can drop the
SDL_image dependency and avoid unnecessary pixel format conversions.