renderer.bounds iterate over a list of actors

[ndelpuppo]

Describe the feature you would like to be added.

When we call the Camera copy() method, the Renderer object calls the bounds() method. Now, if we add actors to the scene using a background concurrent process to preload data, the self._actors dictionary changes size and this generates an error as we iterate over the dictionary values at line 460 in pyvista/plotting/renderer.py:
for actor in self._actors.values():
Would it be possible to change this line to:
for actor in list(self._actors.values()):
?