Fixes to iterator and allocation.
Hack to makt it compile. FIXME
Rewrite halfSample to ditch pointers.

UNTESTED! Test this and compare speed to old version!
Rewrite convert_rgb_to_y.cc to avoid pointers.

Untested but looks rather trivial.
Fix gradient to use indices.

Not apparent if it does entirely the correct thing with scalar conversion...
Fix zero()

This raises some rather interesting questions about what zeroing an image
is. I think perhaps this function should be removed.
More fixes for lack of contiguous images.