Mark 512x400 mode as decent quality.

I originally flagged this as a poor quality mode because it doesn't
include every original pixel at least once (ie. its dimensions are
not >= 640x400 - twice the original resolution). In practise, it's
actually okay though.

Add long comment adapted from my comment on #339 explaining what
the definition of a poor quality mode is.
Add BFG edition workaround for options menu crash.

The BFG edition IWADs have their M_GDHIGH lumps changed to say
"Fullscreen:" instead of just "high". This breaks the options menu
which assumes a graphic that is not as wide.

In the same spirit as the title screen workaround, add a workaround
for this as well: use the message "on/off" graphics instead when we
are running using a BFG edition IWAD.

This fixes #341 (thanks Fabian Greffrath).