tiffoutput checkpointing bugfix
tiffoutput checkpointing bugfix

On large images, the header was being re-written potentially thousands of times.
(resulting in gb of extra writes on large images).  We now throttle write to
only checkpoint at regular intervals.

YangYangTL/oiio,cwilling/oiio,jeremyselan/oiio,bdeluca/oiio,scott-wilson/oiio,sambler/oiio,micler/oiio,cwilling/oiio,bdeluca/oiio,sambler/oiio,micler/oiio,OpenImageIO/oiio,scott-wilson/oiio,bdeluca/oiio,mcanthony/oiio,mcanthony/oiio,lgritz/oiio,YangYangTL/oiio,scott-wilson/oiio,mcanthony/oiio,micler/oiio,mcanthony/oiio,sambler/oiio,OpenImageIO/oiio,sambler/oiio,bdeluca/oiio,YangYangTL/oiio,scott-wilson/oiio,cwilling/oiio,cwilling/oiio,OpenImageIO/oiio,OpenImageIO/oiio,jeremyselan/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,YangYangTL/oiio,jeremyselan/oiio,micler/oiio