Add max samples accessor for WebGLRenderingContextBase It would be used in the default framebuffer management code.
<rdar://problem/117903868>
Pull request: https://github.com/WebKit/WebKit/pull/19948
Committed 270317@main (a48a890fc2ef): <https://commits.webkit.org/270317@main> Reviewed commits have been landed. Closing PR #19948 and removing active labels.