stereo image genration improvement: fix that right side of heads get overpainted by the background

[OWKenobi]

I discovered the depthmap script today, and I am really impressed!

the depth-maps come out really great!

But creating a stereo image from these seems to have a minor issue: take a portrait of a person. On the left side of the head, the background moves to the left, so some pixels have to be "invented". This works quite well. But on the right side of the head, the background also moves to the left, partially covering the head, so part of the face and neck are overwritten - which is wrong! The background should move *behind" the head, not in front of it.

This is fixable, though; if you process images from left-to-right, just remember the height value of the last pixel you generated. Now, if a left shift occurs, and its height value on the new position is actually lower than the pixel already there, don't render it.

Hope someone is able to put this to code :-)