No initial value set for camera_world_position

[rdb]

When directly using base.graphicsEngine.renderFrame() after simplepbr init (eg. to force a single frame to be rendered during a loading screen), an error is shown regarding a missing camera_world_position input.

To work around this problem, the task manager can be stepped.

It would be good if simplepbr.init applied a default value for this variable so that this use case does not result in an error.