Remove deep copy of ImageBuffer in tiled SVG backgrounds
https://bugs.webkit.org/show_bug.cgi?id=110549

Reviewed by Dirk Schulze.

SVGImage::drawPatternForContainer creates a temporary ImageBuffer for stamping out tiled
backgrounds. Previously this ImageBuffer was unnecessarily deep-copied! This patch uses a
shallow copy, taking advantage of the fact that the temporary ImageBuffer is never modified.

This patch is covered with existing tests.

* svg/graphics/SVGImage.cpp:
(WebCore::SVGImage::drawPatternForContainer):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143692 268f45cc-cd09-0410-ab3c-d52691b4dbfc