In order to pick up the upstream fixes for several linked bugs and more, it's necessary to roll ANGLE forward. Attempting that today.
Created attachment 399987 [details] Patch
Note that there are important steps to take when updating ANGLE. See http://trac.webkit.org/wiki/UpdatingANGLE
Created attachment 399990 [details] Patch
BTW, built and tested minimally locally with: https://webglsamples.org/aquarium/aquarium.html the test cases from Bug 210994: https://www.shadertoy.com/view/lltSR8 https://www.shadertoy.com/view/XtVXzV and a couple of other pages.
<rdar://problem/63517568>
Committed r262036: <https://trac.webkit.org/changeset/262036> All reviewed patches have been landed. Closing bug and clearing flags on attachment 399990 [details].