test: percy snapshot workaround (#1111)

* Try workaround for percy snapshot

* Return our modified document

* test: some dark magic to make it possible to take screenshots of WebGL canvases

* Apply transformation directly to test DOM

* investigate: window:before:load

* Register flag in correct place; add experimental-webgl check

* Remaining canvas fixes

* Remove comment

platform/viewer/src/components/ViewportGrid/ViewportGrid.js

@@ -91,6 +91,7 @@ const ViewportGrid = function(props) {
 
   return (
     <div
+      data-cy="viewprt-grid"
       style={{
         display: 'grid',
         gridTemplateRows: `repeat(${numRows}, ${rowSize}%)`,