`DCtx::try_clone` (`experimental` feature, deprecated) is a plain copy of the
C context. A context owning a dictionary (from `load_dictionary`,
`ref_prefix` or `init_using_dict`) or a dictionary table (`ref_ddict` with
`RefMultipleDDicts`) ended up with both copies freeing it. A context cloned
mid-frame resumed on uninitialized stream state. The clone also silently lost
the `WindowLogMax` limit.
The flaw was corrected by refusing to clone such contexts, and by carrying
over the parameters the copy leaves out.