Creates new subclass for HostWindow 'GraphicsClient', for the graphics specific subset. This currently only has createImageBuffer, but in the future will have other graphics-related WebKit functionality (like creating a WebGL context).
<rdar://problem/102730747>
Pull request: https://github.com/WebKit/WebKit/pull/6875
Committed 257096@main (0192c42f090a): <https://commits.webkit.org/257096@main> Reviewed commits have been landed. Closing PR #6875 and removing active labels.