Povray: Fix bunch of issues after recent refactor.

Those were breaking automated tests, since (un)registering was not
working properly. Mainly:

* Use `hasattr` instead of try/except when batch-assigning to a variable
  that does not always exist.
* Fix some incorrect call to sub-`register()` from parent `unregister()`
  functions.
* Remove double definition (in two different files) of World's pov
  textures.