Fixed zips compression, again.

It looks like the previous fix was setting the header to report "zips"
but actually doing no compression at all.  This is for stupid nuke. See
issue 4029.
Merge branch 'master' of github.com:aghiles/microexr

Conflicts:
	src/exrfile.cpp
	src/scanlineblock.h
	src/scanlinezipblock.h

--

This merge was necessary because of overlapping work between Victor
and Olivier. (Regrading wrong number of block entries in the index
table when using ZIP16 compression and other small details).

I favored Olivier's code since it was slightly cleaner.
Small mistake during last merge.