Since this surface is created by initBG instead of loaded from skin
search paths, it didn't really fit in SurfaceCollection. After removing
it, one of SurfaceCollection's methods could be removed as well.
There are a few exclusive operations for each type. Also we no longer
need the freeWhenDone flag since the class now determines whether the
surface should be freed or not.
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.
