Fix iframe hiding to avoid breaking stl, pdb, and images

 * Switch to using `opacity: 0.0` instead of `display: none` to hide
   iframes that are still being built.  Use of `display: none` was
   causing OpenGL-based renderers (pdb and 3D-object) to fail, and was
   disabling image zoom for large images.

Author: NyanHelsing

mfr/server/static/css/mfr.css

@@ -42,14 +42,16 @@ This CSS was stripped out of base.css from the osf-style repo.
     align-items: center;
     width: 100%;
     height: 250px;
+    top: 0;
+    left: 0;
 }
 
 .embed-responsive-pdf {
     padding-bottom: 95%;
 }
 
 #mfrIframe iframe {
-    display: none;
+    opacity: 0.0;
 }
 
 #mfrIframe {

mfr/server/static/js/mfr.js

@@ -82,7 +82,7 @@
             self.pymParent.el.appendChild(self.spinner);
             $(self.pymParent.iframe).on('load', function() {
                 self.pymParent.el.removeChild(self.spinner);
-                this.style.display = "block"
+                this.style.opacity = "1.0"
             })
 
             self.pymParent.onMessage('embed', function(message) {