Preserve the drawing buffer by default for WebGL layers

test/browser/spec/ol/renderer/webgl/PointsLayer.test.js

@@ -25,6 +25,7 @@ const baseFrameState = {
   layerStatesArray: [{}],
   layerIndex: 0,
   pixelRatio: 1,
+  renderTargets: {},
 };
 
 const simpleVertexShader = `
@@ -664,6 +665,7 @@ describe('ol/renderer/webgl/PointsLayer', function () {
             zIndex: 0,
           },
         ],
+        renderTargets: {},
       };
     });
 

test/browser/spec/ol/renderer/webgl/TileLayer.test.js

@@ -51,6 +51,7 @@ describe('ol/renderer/webgl/TileLayer', function () {
       usedTiles: {},
       wantedTiles: {},
       tileQueue: new TileQueue(VOID, VOID),
+      renderTargets: {},
     };
   });