ImageBufferIOSurfaceBackend should flush only when GraphicsContextCG has drawn
<rdar://problem/106192612>
Pull request: https://github.com/WebKit/WebKit/pull/10995
Committed 262010@main (9e8259e401be): <https://commits.webkit.org/262010@main> Reviewed commits have been landed. Closing PR #10995 and removing active labels.