Blender Git Loki

Geometry Nodes: Expose Bezier handle positions as an attribute

This commit exposes left and right bezier handles as an attribute.
Interaction basically works like edit mode. If you move an aligned
handle, it also moves the opposite handle of the control point.
The difference is that you can't edit "Auto" or "Vector" handles,
you have to first use the "Set Handle Type" node. That gives the handle
types a bit more meaning in the node tree-- changing them in edit mod
is more like a "UI override".

The attributes are named `handle_start` and `handle_end`,
which is the same name used in the curve RNA API.

A new virtual array implementation is added which handles the case of
splines that don't have these attributes, and it also calls two new
functions on `BezierSpline` to set the handle position accounting
for aligned handles.

The virtual arrays and attribute providers will be refactored
(probably templated) in the future, as a next step after the last
built-in curve attribute provider has landed.

Differential Revision: https://developer.blender.org/D12005