Renames get_parent_spatial() to get_parent_node3d() and changes its
implementation. Before it was not returning a correct pointer if the
node wasn't added to a SceneTree. Now it uses the same implementation as
CanvasItem, which will be correct even for nodes outside a SceneTree.
This commit adds the following properties to GeometryInstance3D: `visibility_range_begin`,
`visibility_range_begin_margin`, `visibility_range_end`, `visibility_range_end_margin`.
Together they define a range in which the GeometryInstance3D will be visible from the camera,
taking hysteresis into account for state changes. A begin or end value of 0 will be ignored,
so the visibility range can be open-ended in both directions.
This commit also adds the `visibility_parent` property to 'Node3D'.
Which defines the visibility parents of the node and its subtree (until
another parent is defined).
Visual instances with a visibility parent will only be visible when the parent, and all of its
ancestors recursively, are hidden because they are closer to the camera than their respective
`visibility_range_begin` thresholds.
Combining visibility ranges and visibility parents users can set-up a quick HLOD system
that shows high detail meshes when close (i.e buildings, trees) and merged low detail meshes
for far away groups (i.e. cities, woods).
