Minor ol.geom.Circle clean-ups

This PR cleans up a couple of things in `ol.geom.Circle`:

* It corrects the radius calculation in `ol.geom.Circle#containsXY`. A circle is represented by two points: its center and an arbitrary control point on its circumference. This allows a circle to maintain its dimensions even when transformed or reprojected. The previous code assumed that the control point's x coordinate was always zero, which is not necessarily the case if the circle has been reprojected.

* Two `Math.sqrt`s are elminated in `containsXY`.