Avoid heap-allocating a new BackingStoreFlusher for each RemoteLayerTreeDrawingArea::updateRendering() call. Reuse the same BackingStoreFlusher for each call since flushes cannot happen in parallel.
Pull request: https://github.com/WebKit/WebKit/pull/18721
Committed 268962@main (152b2c430626): <https://commits.webkit.org/268962@main> Reviewed commits have been landed. Closing PR #18721 and removing active labels.
<rdar://problem/116557105>