Move RenderSVGImage's paint code to SVGImagePainter

This patch moves RenderSVGImage's paint functions to SVGImagePainter.
This is a straight code refactor with no changes other than exposing a
few functions on RenderSVGImage.

The one hairy part of this patch is the buffered rendering support
we have in RenderSVGImage. This has been moved to a static function
on SVGImagePainter and all callers have been updated.

BUG=412088

Review URL: https://codereview.chromium.org/603053002

git-svn-id: svn://svn.chromium.org/blink/trunk@182704 bbb929c8-8fbe-4397-9dbb-9b2b20218538