add-line's behavior does not match the documentation

[schanzer]

According to the documentation, these two lines of code should have the line starting at the same coord, and ending at a different place. That doesn't happen.

add-line(circle(20, "outline", "maroon"), 0, 40, 40, 0, "orange")
add-line(circle(20, "outline", "maroon"), 0, 40, 20, 0, "orange")

[asolove]

From a bit more playing around with this, it seems that what we're doing is something like drawing the first picture, then drawing a line as a second picture, and then overlaying them at the center point of the two images.

This example might make it clearer:

The second line takes up less space in x and y directions, so the line's x1=0 and y1=0 isn't the same spot relative to the circle as the first line.

[blerner]

Pretty much, yeah. #580 has a (not-yet-tested) fix