Blender Git Loki

Insert keyframes while preserving shape of curve

Apply the De Casteljau algorithm to split the B�zier curve at the X
coordinate where the new key is inserted, and uses the result to update
both the newly inserted and surrounding handles.

For curves that use Auto keyframes this has been largely addressed by
the new algorithm from D2884. This commit extends this to non-auto
handles.

This code is heavily based on D3172 by Alexander Gavrilov (@angavrilov).

Manifest Task: https://developer.blender.org/T81353