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.
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git-svn-id: http://core.svn.wordpress.org/trunk@25777 1a063a9b-81f0-0310-95a4-ce76da25c4cd