[ftinspect] Fix named instances switching.

* src/ftinspect/engine/mmgx.cpp, src/ftinspect/engine/mmgx.hpp:
  Load the default values of MMGX axes from the named instance instead of
  the global default one.

* src/ftinspect/engine/engine.cpp, src/ftinspect/engine/engine.hpp:
  Switch the named instance using `FT_Set_Named_Instance` when loading
  fonts.