Twenty Fourteen: use SVG images instead of CSS3 gradients for featured image background pattern. Props iamtakashi.

 * Fixes laggy loading in Firefox, see #25600.
 * Significantly reduces CPU usage.
 * Scalable and crisp on HiDPI/retina screens.
 * SVG is support in IE9 which doesn't support CSS gradients.

git-svn-id: https://develop.svn.wordpress.org/trunk@25865 602fd350-edb4-49c9-b593-d223f7449a82