Remove all occurences of VR.render() called right before VR.save()

[Xarthisius]

This PR removes unnecessary and very costly .render() call, since Scene.save() explicitly calls Scene.render().

[Xarthisius]

@yt-fido test this please

[chrishavlin]

related: pull request #2743

the two PRs are compatible, but note that after 2743 is merged, doc/source/cookbook/sigma_clip.py could use render=False in the second and third calls to sc.save() since the sigma_clip is applied after rendering.

[munkm]

I totally agree with removing the .render() methods that are unnecessary in our test framework. I do think we don't have it clearly documented on the user side (unless I missed it) that the .save() method also calls render. Maybe adding a cell to our volume rendering tutorial would be a good place for to compare using .render + .show vs. .save? I think this would also help compliment the issue raised by #2733.

[Xarthisius]

@yt-fido test this please

[matthewturk]

@Xarthisius can you rebase or merge with master so we can run the tests?