It was introduced to fix flickering when we didn't wait for previous frame to be displayed to render the next one. Now, two textures should be enough.
Pull request: https://github.com/WebKit/WebKit/pull/15137
Committed 265352@main (241e71855ab5): <https://commits.webkit.org/265352@main> Reviewed commits have been landed. Closing PR #15137 and removing active labels.
<rdar://problem/111100345>