Geometry Nodes: add Voxel Index node

This node gives access to the integer coordinates of the the voxel that is
currently being evaluated by a field. It can be used together with e.g. the
Integer Math and Sample Grid Index node to sample neighboring voxel values.
Previously, one could only get the position of the voxel in object space.

Since sometimes field are evaluated on tiles of many voxels, just having the
voxel coordinates can be misleading. Therefore, this same node also outputs
whether it is a tile and the extent of the tile (which is 1 for normal voxels).

Pull Request: https://projects.blender.org/blender/blender/pulls/147268

scripts/startup/bl_ui/node_add_menu_geometry.py

@@ -911,6 +911,7 @@ class NODE_MT_gn_volume_read_base(node_add_menu.NodeMenu):
         layout = self.layout
         self.node_operator(layout, "GeometryNodeGetNamedGrid")
         self.node_operator(layout, "GeometryNodeGridInfo")
+        self.node_operator(layout, "GeometryNodeInputVoxelIndex")
 
         self.draw_assets_for_catalog(layout, self.menu_path)