fix(mode/rotate): polygonHull sometimes returns array of length 2

[hackily]

d3_polygonHull() sometimes returns an array of length 2, which d3_polygonCentroid() cannot handle.

This adds a check, and falls back to using geoVecInterp().

This fixes #6977

[bhousel]

Great catch @hackily! Thanks for the fix, and welcome! 👋

[bhousel]

@hackily For fixes like this, can you target the 2.x branch? We're using master for development of iD v3, which won't be available for a while.

I ended up cherry picking the fix onto the 2.x branch and merging, that's why GitHub doesn't show it as merged.