Prevent Chrome from selecting off-screen hot spot with tab (#1120).

src/js/pannellum.js

@@ -1856,6 +1856,7 @@ function createHotSpot(hs) {
     hs.yaw = Number(hs.yaw) || 0;
 
     var div = document.createElement('div');
+    div.tabIndex = -1;
     div.className = 'pnlm-hotspot-base';
     if (hs.cssClass)
         div.className += ' ' + hs.cssClass;