which just draw that one eye's view.  Allows easy
stereo screen snapshotting.

Important: changed sign of crosseyed stereo!
This means that positive stereo-separation values
are now correct for crosseyed (as opposed to walleyed) viewing.
To mark this incompatible change, bumped VERSION to 0.7.

Once we pick a license this might change, but
in the mean time, at least the NCSA UIUC origin is noted.

and let postdraw return an int.  Keep looping, re-rendering,
until postdraw returns zero.

so we can use same function for both.

have "o2w" transforms that are really interpreted relative to camera space.
Also start adding per-object clip support; this requires change to
subcameras, supplying projection parameters subc_lrbt()
rather than a (clip-dependent) projection matrix.