TIFF: fix read problem with tiff files with non-zero y offset (#3419)
TIFF: fix read problem with tiff files with non-zero y offset (#3419)

You'd think there would be no way that basic failure like this could
occur after 14 years, right?

But the TIFF reader code was a little mixed up about the fact that the
internal m_next_scanline field is relative to the starting scanline of
the image, not an absolute value. But in the case of the code that has
to deal with handling the stray scanlines at the end of a strip-read
file neglected to set it properly and the end of the stripped part.

Obviously, for TIFF files with a first scanline of 0 (the vast, vast
majority), failing to adjust by this offset doesn't matter. And also
as long as the reading continued to be strictly sequential, it also
was asymptomatic. But in the presence of certain multithreaded random
access patterns, it makes a difference. Oops.
lgritz/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio