These issues are fixed:
* loadPNG gave RGBA surfaces unconditionally. Now it gives RGB surfaces if
its second parameter, defaulting to true, is false. This fixes adding per-
surface alpha values, because an RGBA surface ignores its per-surface value.
* Surface::loadImage is updated to receive a third parameter, defaulting to
true, to determine whether alpha is loaded.
* SurfaceCollection::defaultAlpha was never used.
* SurfaceCollection::defaultAlpha defaulted to false, so even if it were used,
it would have loaded images without alpha.
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.
