Add canvas image content support (#20)

* Add canvas image content support

* use rr_dataURL as the key so we know its a rrweb-added attribute.

* fix canvas rr_dataURL rebuild

src/snapshot.ts

@@ -224,6 +224,10 @@ function serializeNode(
           attributes.selected = (n as HTMLOptionElement).selected;
         }
       }
+      // canvas image data
+      if (tagName === 'canvas') {
+        attributes.rr_dataURL = (n as HTMLCanvasElement).toDataURL()
+      }
       if (needBlock) {
         const { width, height } = (n as HTMLElement).getBoundingClientRect();
         attributes.rr_width = `${width}px`;