Fix that 3d view broke if it had no width

[daniel-wer]

The condition that checked whether the 3d view was not visible, to avoid updating the camera state in that case, was too unspecific. It only checked whether the 3d view had 0 height (and not straightforward, but only by checking whether the result of a division was NaN), but not whether it had 0 width. I changed the condition to be more explicit and complete.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• Easier reproduction instructions than in the issue
• Move the vertical splitter (from between the viewports) all the way to the right so that the 3d view is no longer visible.
• Move it back -> the 3d view content should be visible again (on master it is not)

Issues:

• fixes Reducing size of 3d Viewport to (0,0) crashes it. #5472

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• Updated (unreleased) changelog
• Ready for review

[philippotto]

works great and the code looks better, too 🚀