Codechange: Don't use SDL_CreateRGBSurfaceWithFormat()

This function requires libSDL 2.0.5 or higher. It looks like we don't
need to use it, and can just use the original SDL_CreateRGBSurface(),
with the masks set to 0, to trigger the default 8-bit format, which is
SDL_PIXELFORMAT_INDEX8.

Closes #7785

Note: this code path is activated by using an 8-bit blitter, like:

./bin/openttd -b 8bpp-simple