Change tile & file limit assertions from ASSERT to DASSERT.
Change tile & file limit assertions from ASSERT to DASSERT.

We realize how a particular combination of multithread, low cache memory
limits, and big tiles / untiled can inadvertently trigger these
assertions. Let's turn it into a DEBUG-only assertion for now, so it
won't happen for release builds.

As a more long-term plan, we should probably automatically bump up the
cache limits any time we encounter a tile that is itself bigger than
the memory limit, to avoid hitting this impossible situation.

For detailed discussion, see https://github.com/OpenImageIO/oiio/issues/1239

Fixes #1239

micler/oiio,OpenImageIO/oiio,lgritz/oiio,micler/oiio,micler/oiio,OpenImageIO/oiio,YangYangTL/oiio,lgritz/oiio,OpenImageIO/oiio,bdeluca/oiio,YangYangTL/oiio,lgritz/oiio,bdeluca/oiio,bdeluca/oiio,lgritz/oiio,YangYangTL/oiio,bdeluca/oiio,YangYangTL/oiio,OpenImageIO/oiio,micler/oiio