Arrow angle measurement not working for all arrows

[crangelsmith]

@crangelsmith The issue with above image (238) and most likely others is that the 'bar' forming the tail of the arrow is too short (see image), and is reversing the points (blue and red) that the k-means clustering is using to indicate end-points. It is certainly not the case for other arrows, but it means in the current form, arrow measurement will not work on all arrows.

Originally posted by @JasonGellis in #75 (comment)

[crangelsmith]

Discussions about arrow measurement in this issue #69