Geometry Nodes: Attribute Fill Node

This commit adds a node that fills every element of an attribute
with the same value. Currently it supports float, vector, and color
attributes. An immediate use case is for "billboard" scattering.

Currently people are using the same input to a Random Attribute node's
min and max input to fill every element of a vector with the same value,
which is an unintuitive way to accomplish the same thing.

Differential Revision: https://developer.blender.org/D9790

release/scripts/startup/nodeitems_builtins.py

@@ -483,6 +483,7 @@ geometry_node_categories = [
     GeometryNodeCategory("GEO_ATTRIBUTE", "Attribute", items=[
         NodeItem("GeometryNodeRandomAttribute"),
         NodeItem("GeometryNodeAttributeMath"),
+        NodeItem("GeometryNodeAttributeFill"),
     ]),
     GeometryNodeCategory("GEO_COLOR", "Color", items=[
         NodeItem("ShaderNodeValToRGB"),