Show fullscreen button iff fullscreen is allowed.

src/js/pannellum.js

@@ -169,7 +169,8 @@ container.appendChild(controls.zoom);
 controls.fullscreen = document.createElement('div');
 controls.fullscreen.addEventListener('click', toggleFullscreen);
 controls.fullscreen.className = 'pnlm-fullscreen-toggle-button pnlm-sprite pnlm-fullscreen-toggle-button-inactive pnlm-controls pnlm-control';
-container.appendChild(controls.fullscreen);
+if (document.fullscreenEnabled || document.mozFullScreenEnabled || document.webkitFullscreenEnabled)
+    container.appendChild(controls.fullscreen);
 
 // Compass
 var compass = document.createElement('div');