Here's a speed/memory tradeoff that would help me a lot if I was able
to choose it: currently, the primitive that builds an image segment
requires a pre-allocated WordArray.  If you're creating an image
segment of a significant portion of the data in the image, this
effectively doubles your image size (or more, because it
over-allocates).  It would be great to have a version of this
primitive that used a file instead.  I realize that there's a lot of
random access of the buffer that goes on, which means that a
file-based prim is going to be a *lot* slower than the memory based
one, but in some cases I'm willing to pay that price to keep the image
size low.

If someone is up for writing this prim, I'm happy to compensate them
for their time.

Cheers,
Avi