Clarify comments and add ROI::All()
Refactor computePixelHashSHA1 for parallelism:
1. Allow ROI to designate a section of the image.
2. Option to hash in blocks, then return hash of the hashes (versus hashing
   the entire image as one block).
3. When hashing in blocks, this can be multithreaded!

Change maketx to use the block-based hashing, in blocks of 256 scanlines.
This will be incompatible with old maketx-generated hashes (both will be
accepted by the texture system, but old will not match against new -- the
hashes are different) but significanly speeds up maketx on multi-core
machines.