if we have a single value for roundrect radius, manually clamp it before drawing.

Skia will clamp the radius independently in X and Y to ensure it is not larger than 1/2 the width (or height).
If the caller to our drawable gives us a single value, we'll assume they want it to be circular, and not an oval.
To do that, we clamp it up front, so we get (possibly smaller) circular corners, rather than potentially
elliptical ones.

This makes the progress bar look "nicer" when it is very thin in one dimension.