Fix incorrect mirror formula
Wrap mode for ImageBuf iterators allows them to retrieve valid data even
when having run off the edge.  Select from black, clamp, periodic, mirror.
This makes iterating over ImageBufs with overlap much easier.
Test IB iterator wrap modes
Cleanup of getpixel/getchannel variants, including adding wrap mode
Retire 'unclamped' flag to Iterator constructors.
Now the iterators take their ranges literally; users are free to clamp
to the pixel data window before specifying an iteration range if that's what
they really want.
Coalesce redundant wrap functions in TS and IB.
Merge pull request #531 from lgritz/lg-wrap

Iterators with wrap