circular images, clip css basic shapes

[Fil]

support for CSS basic shapes in the mark’s clip option

the r option of the image mark sets a default width = 2*r, and a default clip=circle()

todo:

• variable r, associated with the r scale
• complete documentation

closes #1413

[mbostock]

Hmm, I wasn’t thinking we would expose the full functionality of CSS clip-path. (Though this isn’t allowing the path(…) syntax that clip-path supports.) I was just thinking of r for the image mark.

[Fil]

OK to keep it internal and limited to the image mark. The ellipse test was just a test :)

[Fil]

superseded by #1425