Z buffering on the 4D

[Steve Dempsey]

I have just discovered that the "zclear()" function only affects the portion
of the z-buffer defined by the current viewport.  Thus it behaves analogously
to "clear()".  The manual entry is ambiguous on this point.  It states:

	zclear loads the z-buffer with the largest possible positive integer.

I originally made the assumption that zclear affected the entire z-buffer,
resulting in some entertaining, but frustrating images.