[sdf] Add shortest distance finding functions.

* src/sdf/ftsdf.c (get_min_distance_line, get_min_distance_conic,
get_min_distance_cubic): New functions.  Note that
`get_min_distance_conic` comes with two implementations (using an
analytical and an iterative method, to be controlled with the
`USE_NEWTON_FOR_CONIC` macro).
2 files changed