Clean up and fix glyph contour generation nits. (issue 566080043 by address@hidden)

[hanwenn]

Reviewers: lemzwerg,

Message:
Werner, can you confirm that this looks correct? I'm especially not
certain about the wrapping.



Description:
Clean up and fix glyph contour generation nits.

There are two bugs:

* Glyphs with multiple contours (e.g. "O" or "i") should compute the
  contours separately

* Contours can wraparound to the start. This leads to OOB reads and/or
  incorrect outlines in other circumstances.

Please review this at https://codereview.appspot.com/566080043/

Affected files (+70, -45 lines):
  M lily/freetype.cc