Use correct circle canvas size

This PR addresses the issue reported in #2041.

#2040 and https://github.com/elemoine/ol3/commit/562d74dbed3043050e772b6bf95c8dc34ea253ec have revealed a bug in ol.style.Circle, where the size of the circle canvas and this.size_ may not match because of the rounding Canvas applies when setting the canvas width and height.

This PR addresses the issue by re-reading the canvas size for determining the actual size of the circle.

Please review.