Reset encoding after insets and environments also for LuaTeX with TeX fonts.

Reset encoding after insets and environments also for LuaTeX with TeX fonts.

With inputenc == "auto" or "default", the encoding changes with
the language and must be reset after an eventual language switch in insets
or environments (see #6216).

However, whether we need to do this does not depend on 8-bit TeX vs. LuaTeX
but on the possible use of more than one encoding for the document.
With "nonTeXFonts", the encoding is utf8,
LuaTeX with TeX fonts requires encoding handling similar to 8-bit TeX.

(Additionally, the value of "params.inputenc" could be tested: if it is
not "auto" or "default", we have just one common encoding and could skip
the reset as well.) Not sure how much time this saves, though.