Avoid using CanvasBase::buffer() when accessing drawing buffer contents for inspector and painting
<rdar://problem/128677907>
Pull request: https://github.com/WebKit/WebKit/pull/29047
Committed 279437@main (5ab485a4c7c8): <https://commits.webkit.org/279437@main> Reviewed commits have been landed. Closing PR #29047 and removing active labels.