Revert pull request #74 since it broke the display of hot spot tooltips.

src/css/pannellum.css

@@ -282,7 +282,7 @@ body {
     position: absolute;
     height: 26px;
     width: 26px;
-    display: none;
+    visibility: hidden;
     cursor: default;
     border-radius: 13px;
     vertical-align: middle;
@@ -300,7 +300,7 @@ body {
 }
 
 div.pnlm-tooltip span {
-    display: none;
+    visibility: hidden;
     position: absolute;
     border-radius: 3px;
     background-color: rgba(0,0,0,0.7);
@@ -313,7 +313,7 @@ div.pnlm-tooltip span {
     cursor: default;
 }
 div.pnlm-tooltip:hover span{
-    display: block;
+    visibility: visible;
 }
 div.pnlm-tooltip:hover:after {
     content: '';

src/js/pannellum.js

@@ -1078,9 +1078,9 @@ function renderHotSpots() {
         var z = hsPitchSin * configPitchSin + hsPitchCos * yawCos * configPitchCos;
         if ((hs.yaw <= 90 && hs.yaw > -90 && z <= 0) ||
           ((hs.yaw > 90 || hs.yaw <= -90) && z <= 0)) {
-            hs.div.style.display = 'none';
+            hs.div.style.visibility = 'hidden';
         } else {
-            hs.div.style.display = 'block';
+            hs.div.style.visibility = 'visible';
             // Subpixel rendering doesn't work in Firefox
             // https://bugzilla.mozilla.org/show_bug.cgi?id=739176
             var transform = 'translate(' + (-renderer.canvas.width /