[sdf] Use 8 bits for final SDF output instead of 16bits.

Since 8-bits is enough to represent SDF data we no longer require
16-bits for  this purpose. Also, we now normalize the output data
to use the entire 8-bit range efficiently. For example: if we use
3.5 format with a spread of 1 we basically only use the starting
5-bits. By normalizing we can use the entire 8-bit range.

* include/freetype/freetype.h (FT_Render_Mode): Updated description
for `FT_RENDER_MODE_SDF` regarding this change.

* include/freetype/ftimage.h (FT_Pixel_Mode): Removed
`FT_PIXEL_MODE_GRAY16` since no longer required.

* include/freetype/fttypes.h (FT_F6Dot10): Removed since no longer

* src/sdf/ftsdfrend.c (ft_sdf_render, ft_bsdf_render): Allocate 8-bit
bitmap instead of 16-bit buffer.

* src/sdf/ftsdfcommon.h (map_fixed_to_sdf): Added function to convert
16.16 distance value to our desired format.

* src/sdf/ftsdf.c (sdf_generate_with_overlaps,
sdf_generate_bounding_box): Use the new `map_fixed_to_sdf` function
and also use 8-bit output buffer.

* src/sdf/ftbsdf.c (finalize_sdf): Output to a 8-bit buffer instead
of 16-bit buffer.
8 files changed