Add canvas checks

src/ol/canvas/canvas.js

@@ -0,0 +1,21 @@
+goog.provide('ol.canvas');
+
+goog.require('goog.dom');
+goog.require('goog.dom.TagName');
+
+
+/**
+ * @return {boolean} Is supported.
+ */
+ol.canvas.isSupported = function() {
+  if (!('HTMLCanvasElement' in goog.global)) {
+    return false;
+  }
+  try {
+    var canvas = /** @type {HTMLCanvasElement} */
+        (goog.dom.createElement(goog.dom.TagName.CANVAS));
+    return !goog.isNull(canvas.getContext('2d'));
+  } catch (e) {
+    return false;
+  }
+};