Editor: fix preview area centering for both canvas and iframe

Author: pepc84

editor/index.html

@@ -69,6 +69,9 @@
       background: #111;
       overflow: hidden;
       position: relative;
+      display: flex;
+      align-items: center;
+      justify-content: center;
     }
     #preview-area canvas { display: block !important; }
     #preview-area iframe { position: absolute; top: 0; left: 0; width: 100%; height: 100%; border: none; display: block; }