Fix incorrect miter clip length

[nickyfahey]

If I'm understanding it correctly MiterClip should clip at a distance of the miter limit multiplied by the stroke width, but at the moment it actually clips to a quarter of this because the half width is being halved again instead of doubled in the clip distance calculation.

Possibly related to #923 but I could not reproduce the bug exactly as described (falling back to bevel as opposed to just getting the clip length wrong).

Before fix:

After fix:

[nical]

Thanks!

According to the SVG spec:

the miter is clipped by a line perpendicular to the line bisecting the angle between the two path segments at a distance of half the value of miter length from the intersection of the two path segments.

So unless I missed something, lyon currently clips at half of the correct distance (oops), and your change makes it clip at twice the correct distance.

The fix should be:

get_clip_intersections(n0, n1, normal, miter_limit * join.half_width);

[nickyfahey]

@nical Woops, looks like I was looking at the SVG 1 spec. Fixed it now.

[nical]

Thanks!