draw bevel linejoins without double-drawing

it now looks much nicer when drawn with transparency.

before:
https://cloud.githubusercontent.com/assets/1421652/3885641/3d2a90f2-21c7-11e4-9fe0-af29c58dabb3.png

after:
https://cloud.githubusercontent.com/assets/1421652/3885642/3d318682-21c7-11e4-961f-56aabf6f76a2.png