[sfnt] Clarify `COLR` v1 FT_Paint* format representations

* include/freetype/ftcolor.h (FT_PaintLinearGradient,
FT_PaintRadialGradient, FT_PaintSweepGradient, FT_PaintTransform,
FT_PaintTranslate, FT_PaintScale, FT_PaintRotate, FT_PaintSkew): Clarify
16.16 fixed point representation of struct fields.
* src/sfnt/ttcolr.c (read_paint): Shift coordinates for
FT_PaintLinearGradient, FT_PaintRadialGradient, FT_PaintSweepGradient
accordingly.

Fixes: https://gitlab.freedesktop.org/freetype/freetype/-/issues/1110
2 files changed