Only clear canvas when necessary, add tests

src/ol/renderer/canvas/tilelayer.js

@@ -228,6 +228,9 @@ ol.renderer.canvas.TileLayer.prototype.prepareFrame = function(frameState, layer
         canvas.width = width;
         canvas.height = height;
       } else {
+        if (this.renderedExtent_ && !ol.extent.equals(imageExtent, this.renderedExtent_)) {
+          context.clearRect(0, 0, width, height);
+        }
         oversampling = this.oversampling_;
       }
     }