Change some fields in `FT_Bitmap' to unsigned type.

This doesn't break ABI.

* include/ftimage.h (FT_Bitmap): Make `rows', `width', `num_grays',
`pixel_mode', and `palette_mode' unsigned types.

* src/base/ftbitmap.c: Updated.
(FT_Bitmap_Copy): Fix casts.

* src/cache/ftcsbits.c, src/raster/ftraster.c, src/sfnt/pngshim.c:
Updated.
6 files changed