Global attribute("read_chunk") controls how many scanlines to try reading
at a time in read_image, so it can be tuned by applications.
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.