[Bug-gnubg] Faster 3d board drawing option

[Jon Kinsey]

I've added a new option which aims to improve the performance of the 3d 
board when playing games on slower machines.  Note that browsing matches 
and other actions won't be any faster.

This is aimed at people with none or slow opengl graphics hardware 
acceleration.  Without hardware acceleration performance is normally 
about 1fps, this isn't usable.  With this new option it might be usable...

I class slow 3d graphics performance as less than about 5fps, in theory 
the improvement here should be the biggest.  In practice these cards may 
have difficulty coping with the new code I've added...

After that "hard-sell" I'll just add that I haven't done much testing or 
finished everything yet, so there's likely to be things not being 
displayed and/or erased correctly in certain circumstances.

Anyway if the 3d performance is bad on your machine give it a try and 
see if selecting "quick draw" in the appearance dialog helps (or works)...

Jon