Update gui.py

[stymbhrdwj]

I was encountering a bug when running the GUI with type "neilf" on the DTU dataset. The issue was pc._visibility_tracing was None, which was being indexed in neilf.py in render_view function. I added a call to the update_visibility method in gui.py to fix this.

[caobiru]

@stymbhrdwj Thank you so much! This helped a lot!