Move <svg:image> buffered-rendering management to SVGImagePainter

There are no strong dependencies on SVGRenderingContext or other
surroundings, so the move is reasonably straight-forward.
While moving, modify the flow of the bufferForeground function a bit.

BUG=412088

Review URL: https://codereview.chromium.org/675173003

git-svn-id: svn://svn.chromium.org/blink/trunk@184542 bbb929c8-8fbe-4397-9dbb-9b2b20218538