If loading fails, the factory method returns NULL, while previously the
constructor would create a Surface object with a NULL "raw" field.
However, since most of the methods dereference "raw" without checking,
such a Surface would likely crash the application when used.
The unnecessary alpha channel might harm performance but is not a big deal.
However, the fact that the alpha flag (bool) is somehow automatically
converted by C++ into a skin name (string) causes the entire screen to
stay black.
The black screen was fixed by removing the alpha flag.
For optimum blitting performance, the wallpaper surface is then converted
to the pixel format of the frame buffer.
Bug was introduced in 52c89d6005.
Although exposing a read/write reference to a private field is not much better than having a public field.
Well, at least we are explicit about the nasty stuff now.
