fix(webgl): add osm layer context lost test

Author: BryonLewis

tests/gl-cases/osmLayer.js

@@ -22,6 +22,26 @@ describe('osmLayer', function () {
     imageTest.imageTest('osmLayerVgl', null, 0.0015, done, myMap.onIdle, 0, 2);
   });
 
+  it('lose context and recover', function (done) {
+    myMap = common.createOsmMap({center: {x: -78, y: 21}}, {
+      renderer: 'webgl',
+      attribution: '&copy; <a href="http://some-unvisited-domain.org">OpenStreetMap</a> contributors'
+    });
+    myMap.draw();
+    myMap.center({x: 60, y: 20}).zoom(3);
+    const gl = myMap.node().find('canvas')[0].getContext('webgl');
+    const ext = gl.getExtension('WEBGL_lose_context');
+    // we must wait until the context is available and then until it is lost
+    window.setTimeout(() => {
+      ext.loseContext();
+      window.setTimeout(() => {
+        ext.restoreContext();
+        myMap.center({x: 0, y: 0}).zoom(2.5);
+        imageTest.imageTest('osmLayerVgl', null, 0.0015, done, myMap.onIdle, 0, 2);
+      }, 10);
+    }, 10);
+  });
+
   it('canvas renderer', function (done) {
     myMap = common.createOsmMap({}, {
       renderer: 'canvas',