Allow IB::Iterator ctr that takes an ROI to also take the 'unclamped' flag.
Also some simplification of other constructors.
maketx perf: make copy_block use iterators and specialized types
Change default for memory_used to report resident memory, not virtual memory.
Make Linux properly make the distinction as well (previously only reported VM).
maketx guts overhaul -- switch to shared_ptr<ImageBuf> and be careful about
avoiding extra copies and freeing memory quickly.
Features an overhaul of how we report time and memory use in -v -stats mode.
Speed up sha-1
(a) avoid extra pixel copy for ImageBuf with local pixels.
(b) chunk more than one scanline at a time.
Speed up SHA-1 by using OpenSSL's implementation if available -- ~20% faster.