Figure out the actual problem with canon / axis_len

Rotating the coordinate system is what broke the axis_len calculation,
and a stray rotation was present for the previous commits. The previous
solution doesn't work because rotation could be arbitrary.

This solution does work for any rotation, however. When calculating the
axis length for an arc:
1) calculate a 3D displacement from start to end in XYZ
2) unrotate the displacement vector in the XY plane (i.e. rotate by
-xy_rotation).
3) select the appropriate component by the active plane.
Merge branch 'hotfix/velocity-limits' into circular-blend-arc-rc3