(reporting this rather than just fixing it since I can't figure it out at
the moment and it's low priority compared to the other issues with smooth
that need to be resolved)

With smooth() in P3D, something is going wrong with the zbuffer
computation, causing zbuffer values to start at the same value on the left
edge of each triangle until the starting pixel changes (rather than varying
smoothly as the actual start point varies). Tried to track it down and I
couldn't figure out exactly what's wrong; perhaps some sort of rounding is
taking place or something like that?

This is fairly low impact, as the error is usually pretty small, but it
shows up as a glitch whenever polygons clip against each other, since the
cut line becomes a bit jagged.

noted, thanks.