* src/sdf/ftsdf.c: Include `ftbitmap.h`.
* src/sdf/ftsdfcommon.h: Move function bodies to `ftsdfcommon.c`.
Include `ftobjs.h` to get definitions of `FT_LOCAL` and friends.
* src/sdf/ftsdfcommon.c: New file.
* src/sdf/rules.mk, src/sdf/sdf.c: Updated.
Here is a breakdown of what the files will contain.
* src/sdf/ftsdfrend.c, src/sdf/ftsdfrend.h: The 'sdf' and 'bsdf'
renderers.
* src/sdf/ftsdf.c, src/sdf/ftsdf.h: The rasterizer for the 'sdf'
renderer.
* src/sdf/ftbsdf.c, src/sdf/ftbsdf.h: The rasterizer for the 'bsdf'
renderer.
* src/sdf/ftsdfcommon.h: Commmon properties and functions for both
rasterizers.
* src/sdf/ftsdferrs.h: Common error defines.
* src/sdf/sdf.c: For building a single object of the entire module.