Raster overlays should update when moved between terrain tilesets

[r-veenstra]

When moving a raster overlay from one tileset to another (eg. drag/drop via the stage), the raster overlay appears to be stuck on the original tileset and does not switch to the new one.

Now, this isn't necessarily a bug and it is working as designed, as the relationship between tileset and overlay is via a the Raster Overlay Binding property, and not based on parent/child heirarchy in the stage.

Still, based on our other runtimes behaviour, plus the nesting of the raster overlay under the tileset, users may expect this parent/child relationship to influence what tileset a raster renders on.

Perhaps we could automatically update bindings when a raster overlay is parented/unparented from a tileset prim?