Removing global document references (#1482)

fix for options `recordCanvas: true`: 
* replace document.createElement with doc.createElement in rrweb-snapshot code
* Eoghan: add a regression test to prevent future accidental use of `document` instead of `doc`.  This test can be excised if a new feature can only be run in the browser and not in the jsdom environment

packages/rrweb-snapshot/src/snapshot.ts

@@ -726,7 +726,7 @@ function serializeElementNode(
       );
 
       // create blank canvas of same dimensions
-      const blankCanvas = document.createElement('canvas');
+      const blankCanvas = doc.createElement('canvas');
       blankCanvas.width = (n as HTMLCanvasElement).width;
       blankCanvas.height = (n as HTMLCanvasElement).height;
       const blankCanvasDataURL = blankCanvas.toDataURL(