Pref/rest in Houdini

This fixes our treatment of `Pref` (`rest`) in Houdini by marking it as non-transforming. If you use the default Rest SOP in Houdini, this is the behaviour you will get, and discussing with their developers, they expect the prim var called `rest` to behave this way. Our old behaviour was causing issues for users when using the `SceneCacheSource` in `Flattened` mode, and when extracting that flattened hierarchy back into it's OBJ equivalent using `object_merge` SOPs.

Once merged, I'm going to also push these changes to the `8_maintenance` branch. It does change behaviour, but I think it can be qualified as a bug fix.