Improve handling of color palettes.

This commit adds new functions to streamline palette access:

  FT_Palette_Set: Set a user-defined palette.
  FT_Palette_Get: Get palette and palette index.
  FT_Palette_Get_Foreground_Color: Get foreground color.

Fixes issue #1134.

* include/freetype/internal/tttypes.h (TT_FaceRec): Change type of
`palette_index` to `FT_Int`.  Negative values now represent user-defined

* src/base/ftcolor.c (FT_Palette_Set, FT_Palette_Get,
FT_Palette_Get_Foreground_Color): New functions.

* include/freetype/ftcolor.h: Updated.
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