image should have a `rotate` option just like dot

[llimllib]

It would be really handy for me if image marks had the rotate option just like dot marks do.

My particular use case is making a diamond chart, which requires rotating the whole plot, which necessitates rotating any of the elements that appear within.

[Fil]

I've put up a PR in #1084, feel free to review it. Thanks!

[llimllib]

Awesome! Thanks