Voodoo 5 (tdfx) at 24 bpp and dexter

[Joe Drew <hoserhead@woot.net>]

I've got a Voodoo 5, and dexter defaults to 24 bpp for the tdfx driver,
which doesn't agree with the driver (I don't think it properly supports
anything but 16 bpp at the moment) so I get pretty much nothing but garbage
until I change DefaultDepth to 16 rather than 24. I think this might be
a reasonable default, at least for tdfx.

Also, somewhat off-topic: Has 3dfx sync'ed their trees for XF86 (ie: the
X server on linux.3dfx.com) with the Xfree4 tree? I notice some jittering
in 3d games, etc, and lack of performance, which isn't present in their
XFCOM server.