Document `FT_Outline_Decompose` degenerate segments.

`FT_Outline_Decompose` does not filter out and can return degenerate
segments in an outline. This can be surprising when attemping to
stroke such an outline. Clarify the existing documentation on this
matter to cover all forms of degeneracy (without specifying exactly
how they will be reported), why they might arise, and better explain in
what cases they may be an issue.

* include/freetype/ftoutlin.h (FT_Outline_Decompose): update

Fixes #952.
1 file changed