Enable SVG WebGL textures

This is a merge of http://trac.webkit.org/changeset/157647 by Dean
Jackson <dino@apple.com>.

If the HTMLImageElement passed to texture2D is an SVG image, paint it
first into a bitmap buffer and upload that. Note that the SVG image
still needs to have an intrinsic or explicit size - see how the test
case must set width and height.

This patch also renames the cache of ImageBuffers since it is no longer
only being used for video frames.

BUG=309161

Review URL: https://codereview.chromium.org/29203004

git-svn-id: svn://svn.chromium.org/blink/trunk@160156 bbb929c8-8fbe-4397-9dbb-9b2b20218538