Bounding box computation is wrong for rectangle in document with document units:mm

I've added a failing unit test here:

Looks like e15529f3 broke the BB computation for rectangles by introducing a bad unit conversion.

Before (Dimension extension):

After:

git bisect run pytest tests/test_dimension.py shows that e15529f3 is the offending commit.

It's obviously not limited to this particular extension. I'm working on repairing the scatter extension which is affected as well.