Geometry Nodes: Initial basic curve data support

This patch adds initial curve support to geometry nodes. Currently
there is only one node available, the "Curve to Mesh" node, T87428.

However, the aim of the changes here is larger than just supporting
curve data in nodes-- it also uses the opportunity to add better spline
data structures, intended to replace the existing curve evaluation code.
The curve code in Blender is quite old, and it's generally regarded as
some of the messiest, hardest-to-understand code as well. The classes
in `BKE_spline.hh` aim to be faster, more extensible, and much more
easily understandable. Further explanation can be found in comments in
that file.

Initial builtin spline attributes are supported-- reading and writing
from the `cyclic` and `resolution` attributes works with any of the
attribute nodes. Also, only Z-up normal calculation is implemented
at the moment, and tilts do not apply yet.

**Limitations**
 - For now, you must bring curves into the node tree with an "Object
   Info" node. Changes to the curve modifier stack will come later.
 - Converting to a mesh is necessary to visualize the curve data.

Further progress can be tracked in: T87245
Higher level design document: https://wiki.blender.org/wiki/Modules/Physics_Nodes/Projects/EverythingNodes/CurveNodes

Differential Revision: https://developer.blender.org/D11091

release/scripts/startup/nodeitems_builtins.py

@@ -505,6 +505,9 @@ geometry_node_categories = [
         NodeItem("ShaderNodeSeparateRGB"),
         NodeItem("ShaderNodeCombineRGB"),
     ]),
+    GeometryNodeCategory("GEO_CURVE", "Curve", items=[
+        NodeItem("GeometryNodeCurveToMesh"),
+    ]),
     GeometryNodeCategory("GEO_GEOMETRY", "Geometry", items=[
         NodeItem("GeometryNodeBoundBox"),
         NodeItem("GeometryNodeTransform"),