Allow canvas reuse for WebGL layers

src/ol/source/Raster.js

@@ -16,6 +16,7 @@ import {assign} from '../obj.js';
 import {createCanvasContext2D} from '../dom.js';
 import {create as createTransform} from '../transform.js';
 import {equals, getCenter, getHeight, getWidth} from '../extent.js';
+import {getUid} from '../util.js';
 
 let hasImageData = true;
 try {
@@ -646,6 +647,7 @@ class RasterSource extends ImageSource {
       }),
       viewHints: [],
       wantedTiles: {},
+      mapId: getUid(this),
     };
 
     this.setAttributions(function (frameState) {