Constrain rounded rect radii to half the width & height.

This fixes round rects being rendered like this when they have excessively large radii:
![extreme-radius](https://cloud.githubusercontent.com/assets/887005/2904561/bfbdd2a2-d5fc-11e3-9d29-3dac40faebb3.png)

Per http://www.w3.org/TR/SVG11/shapes.html#RectElement:

    6. If rx is greater than half of ‘width’, then set rx to half of ‘width’.
    7. If ry is greater than half of ‘height’, then set ry to half of ‘height’.