revert dhint check
revert dhint check

We added a check that loader ->header() and ->load() methods set the
same dhint. This commit removes that check -- it's not possible to make
header and load match in all cases since images can take so many routes,
and can change between the two events.

If we see performance issues around inappropriate cache sizing again,
try to fix just those cases.

lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips