[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'.