video: Change default for screen_bpp to 0.

Some machines don't work well with 8-bit screen depths any more. It's
better to default to just using the machine's native color depth
instead. Change the default to 0 (for SDL_SetVideoMode this means "use
native color depth"), auto-adjust to native color depth on startup if
screen_bpp=0 (so that debug messages at least make sense) and document
for the config file value that a value of zero means "use native".