3D external WMS not shown

[davidoesch]

When opening
https://s.geo.admin.ch/8e899f8a50

and switch to 3D

WMS is not shown https://s.geo.admin.ch/8e89a02cbc

expected result WMS shown

Issue in our proxy?

[procrastinatio]

The external images are missing all CORS headers, so they are labelled as tainted and cannot be displayed in 3D.

I don't know why suddenly, so many external sites move away from CORS...

Same as #5229