DeepData: need to make internal alloc() thread-safe
DeepData: need to make internal alloc() thread-safe

The old DeepData required clients to call an explicit deep_alloc, which
they could do serially, after which parallel data accesses were
thread-safe. But the new DeepData doesn't have deep_alloc; instead, the
allocation happens implicitly whenever data needs to be accessed (i.e.,
in the potentially parallel section). So the internals themselves need
to ensure thread safety of the allocation, whenever it happens.

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