Use a cubic bezier approximation for rounded rectangle corners.

According to the SVG spec these corners are supposed to be elliptical arcs.  HTML canvas does not have methods for elliptical arcs, but a cubic approximation can get within 0.02%.

Using the "magic number" from http://itc.ktu.lt/itc354/Riskus354.pdf.
Merge pull request #1314 from rodovich/round-rect-arcs

Use a cubic bezier approximation for rounded rectangle corners.