Set direction: ltr; on the canvases used during printing (bug 1335712)

This essentially mirrors the CSS rules used for the viewer itself, see `web/pdf_viewer.js`, and according to my very quick tests it seems to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1335712

web/viewer.css

@@ -1800,6 +1800,7 @@ html[dir='rtl'] #documentPropertiesOverlay .row > * {
   }
   #printContainer canvas,
   #printContainer img {
+    direction: ltr;
     display: block;
   }
 }