[truetype, snft] Remove rounding and truncating of delta values

Remove rounding and truncating of delta values in tt_var_get_item_delta. Change FT_ItemVarDelta typedef from FT_Int32 to FT_Fixed.

Fixes #1257.

• include/freetype/internal/ftcalc.h (FT_MulAddFix): Change return type to FT_Fixed.
• include/freetype/internal/ftmmtypes.h (FT_ItemVarDelta): Change FT_ItemVarDelta from FT_Int32 to FT_Fixed.
• src/base/ftcalc.c (FT_MulAddFix): Remove rounding and truncating the returned value.
• src/cff/cffdrivr.c (cff_get_item_delta): Update.
• src/sfnt/ttcolr.c (read_paint, tt_face_get_color_glyph_clipbox, tt_face_get_colorline_stops): Update item_deltas values casting.
• src/truetype/ttgxvar.c (tt_var_get_item_delta,tt_hvadvance_adjust, tt_apply_mvar, ft_var_to_normalized): Update deltas values casting.