workaround for NeoMagic X server looping

[Eric Cooper <ecc@cmu.edu>]

In both 4.0.3 and 4.1.0, the X server would spin using 100% of the CPU
on my IBM ThinkPad 600 (NM 2160). Turning on Option "NoAccel" stopped
this, but performance was unacceptable.  None of the various "xaaNo*"
options I found in the archives seemed to help.

Finally, I found that using
	 Option "ShadowFB"
works well.  The man page for shadowfb(4) seems to be missing, so I
have no idea why this works. (I guess it's the general principle that
adding a layer of indirection can fix any problem :-)

Hope this is useful to others.

Eric
ecc@cmu.edu