Fix HTML Canvas typing issues

src/ol/ImageCanvas.js

@@ -94,7 +94,7 @@ ImageCanvas.prototype.load = function() {
 
 
 /**
- * @inheritDoc
+ * @return {HTMLCanvasElement} Canvas element.
  */
 ImageCanvas.prototype.getImage = function() {
   return this.canvas_;

src/ol/dom.js

@@ -10,14 +10,14 @@
  * @return {CanvasRenderingContext2D} The context.
  */
 export function createCanvasContext2D(opt_width, opt_height) {
-  const canvas = document.createElement('CANVAS');
+  const canvas = /** @type {HTMLCanvasElement} */ (document.createElement('CANVAS'));
   if (opt_width) {
     canvas.width = opt_width;
   }
   if (opt_height) {
     canvas.height = opt_height;
   }
-  return canvas.getContext('2d');
+  return /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
 }