[IMP] base: allow 'render' override with new api

New API use a frozendict for context, so when a function in the call
chain is overrided by new API (eg: render function of ir.qweb) the code
now changed by this commit would cause an error.

Instead this code duplicate the context frozendict as a dict removing
the need to set back the intial language in the context.

opw-681167

note: no need for >=saas-11