Add negative index support for image2d.
Indexes are supposed "unsigned" but it's a problem with reindex.
Suppose:
image2d<int> f(10,10,3); // 3 pix border
image2d<int> g(10,10,0); // 0 pix border
reindex(g, f); // fails (start index is negative)
- Remove size_type type in image by index_type