Blender Git Loki

Fix T77330, T81704: Spline IK doesn't preserve bone scale correctly

Previously, the bone position outside of "fit to curve length" mode was
incorrect.

It assumed that the curve was completely straight with no bends or
turns. This would lead to bones being scaled down as their final
position would be servery underestimated in some cases.

The solution is to do a sphere -> curve intersection test to see where
to put the bones while still preserving their length. As we are using
the tessellated curve data this essentially boils down to us doing a
sphere -> line intersection check.

Reviewed By: Sybren

Differential Revision: https://developer.blender.org/D10849