bugfix: add missing CSS for hiddenCanvasElement

This is required by the new pdf.js version.

Author: octocorvus

viewer/css/pdf_viewer.css

@@ -45,3 +45,12 @@ canvas {
 [data-main-rotation="270"] {
     transform: rotate(270deg);
 }
+
+.hiddenCanvasElement {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 0;
+    height: 0;
+    display: none;
+}