Reworking of extension handling in -3dfx..
Support for changing the 3dfx dithering mode, if the extension is supported.
(Extension is /not/ in upstream mesa yet.)
gl_ztrick - here's the hat, but no rabbit
considering that a buggy driver can make gl_ztrick into a sort of cheat if
you can stand the flicker, it's gone. gl_finish was already a zombie Cvar
so I just removed its definition.