autoHeight factors in the auto and manual margins.

[Fil]

See https://observablehq.com/@observablehq/plot-band-scale-bug-1045

fixes #1045

Note: this is the least disruptive I could do; but it becomes a bit complicated to understand, as we need to compute what the default margins would have been, and add the difference.

(also includes #1059, in order to pass CI)

[Fil]

🙏 it's easier to read this way!