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.
Renamed methods that draw a single line from write() to writeLine().
There is now only one write() method left: the public method.
Pass surface to draw on as wrapped Surface instead of SDL_Surface.
At the end of the call chain we still use SDL directly though.
There is no point in precalculating something so cheap to recalculate.
Also, the majority of uses was to compensate for passing the wrong alignment argument.
