Move `CF2_Font' instance to `PS_Decoder'. This is the context for
the interpreter and since it is currently stored in `CFF_Font', is
unavailable in Type 1 mode.
* include/freetype/internal/psaux.h (T1_Decoder, PS_Decoder): New
`cf2_instance' field.
* src/psaux/psdecode.c (ps_decoder_init): Copy `cf2_instance' to
`PS_Decoder'.
* src/psaux/t1decode.c (t1_decoder_done): Add finalization code.
* src/psaux/psft.c (cf2_decoder_parse_charstrings): Update accesses.