fitViewportToNodes does not work with custom bounding box

[jacomyal]

After discussing with @Rich5 (#1471), It seems that the fitViewportToNodes does not properly work when using a custom bounding box (set with sigma#setCustomBBox).

Actions to take:

1. Create a reproduction case for that issue in the StoryBook to validate the problem
2. Also check with other options that could similarly break that interaction (autoRescale setting, for instance)
3. Add a regression test to @sigma/utils
4. Fix the issue