Plot.image rotate option

[Fil]

Demo: https://observablehq.com/@observablehq/plot-image-rotate-1084

closes #1083

todo:

• [ ] unit test
• [ ] dx, dy— & anticipate what will happen if dx and dy become variable

[llimllib]

It looks great to me, thanks for the quick response! Though I have to say I don't understand the transform-origin bit at all

[mbostock]

This looks useful.

[Fil]

superseded by #1426