OpenEXR: Fixed issue where a stringvec of 16 items is mishandled as a maxtrix
fmath.h: Move all the "using" outside the OIIO namespace.

If the 'using' is inside the namespace block, it's basically useless
to apps that include fmath.  This mainly affects functions that fmath.h
supplies because they are missing from certain MSVS versions.
Bug fix: oiiotool color conversion botched inputs with MIP levels.

It should have been looping over the number of subimages and mip levels
in the destination image, not the source image, because if the -a flag
is not used, the dst image will be constructed with fewer mip levels
(i.e. 1) than the source. Looping over the source's count will run off
the end of the list.