handle WebGL layers

src/ol/source/Raster.js

@@ -917,6 +917,9 @@ function getImageData(layer, frameState) {
   }
   const container = renderer.renderFrame(frameState, null);
   let element;
+  if (container instanceof HTMLCanvasElement) {
+    element = container;
+  } else {
     if (container) {
       element = container.firstElementChild;
     }
@@ -927,6 +930,7 @@ function getImageData(layer, frameState) {
       const context = element.getContext('2d');
       return context.getImageData(0, 0, width, height);
     }
+  }
 
   if (!sharedContext) {
     sharedContext = createCanvasContext2D(width, height);