Fix line drawing again, for reals!

Summary:
kmath to the rescue!
Not the prettiest kmath i have written, but at least it's a bit cleaner and
works. angleDeg/angleRad functions might be worthy additions to kvector

Test Plan:
https://ka-perseus-graphie.s3.amazonaws.com/3c21069ce144e46c20c6356dfe1b56d9975418aa.js
https://ka-perseus-graphie.s3.amazonaws.com/e1360753e207013f565783a24b40f20cfb9e8a7e.js

Auditors: alex