Constrain rounded rect radii to half the width & height.
Optimize for common case of radius 0.
Merge pull request #1312 from rodovich/round-rect-radii

Constrain rounded rect radii to half the width & height.
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.