Better method for rendering AA ellipses.

Better method for rendering AA ellipses.

This uses the standard ellipse equation as a signed distance
test, and adjusts the result by the length of the gradient
at that point to get a better approximation of the distance
to the ellipse. It replaces the standard ellipse and
roundrect shader renderers.

Also adds a check to see if the curvature extrema of the
stroke are less than the curvature extrema of the ellipse (i.e.
the radius of curvature is larger). In this case, it's no
longer an ellipse and we can't use this renderer.

R=bsalomon@google.com, robertphillips@google.com

Author: jvanverth@google.com

Review URL: https://chromiumcodereview.appspot.com/14938004

git-svn-id: http://skia.googlecode.com/svn/trunk@9016 2bbb7eff-a529-9590-31e7-b0007b416f81