Add reset rotation key to side-by-side demo

demos/side-by-side/index.html.in

@@ -38,7 +38,7 @@
             </tr>
             <tr>
                 <td>Rotate:</td>
-                <td><code>Alt</code>+drag (WebGL only)</td>
+                <td><code>Alt</code>+drag, <code>r</code> to reset (WebGL only)</td>
             </tr>
             <tr>
                 <td>Brightness/contrast:</td>

demos/side-by-side/side-by-side.js

@@ -65,6 +65,9 @@ keyboardControl.addCallback('o', function() {
 keyboardControl.addCallback('O', function() {
   layer.setOpacity(layer.getOpacity() + 0.1);
 });
+keyboardControl.addCallback('r', function() {
+  webglMap.setRotation(0);
+});
 keyboardControl.addCallback('s', function() {
   layer.setSaturation(layer.getSaturation() - 0.1);
 });