This restores the option of rendering an ERD with an absolute width,
which is consistent with the approach taken with the other diagram
types.
This logic was lost in #1324, which was probably just a small oversight:
9199546dca (diff-7c38d27acbe0676d923bf19283671616L397-L409)
The option defaults to true for backwards compatibility.
This option lets you customise the padding around the whole flowchart.
It is equivalent to `er.diagramPadding` and defaults to 8px for
backwards compatibility with the current hardcoded value.
