[truetype, type1] Additional variation tags.

Sync with
  https://learn.microsoft.com/en-us/typography/opentype/spec/dvaraxisreg#registered-axis-tags

* src/truetype/ttgxvar.h (TTAG_ital): New tag.
* src/truetype/ttgxvar.c (TT_Get_MM_Var): Use it.
* src/type1/t1loaad.c (T1_Get_MM_Var): Handle 'slnt' and 'ital'.
3 files changed