[FIX] web_editor: sign documents with safari 15.4+ with ios

Issue:
=====
Users with IOS can't sign with safari.

Steps to reproduce the issue:
=============================
- Install sign
- Make sure you have a template which has at least one signature sign
  item.
- Share the document and open the link with ios on safari in v 15.4+
  (browserstack) for example.
- Go to the signature sign item and use auto and click sign or sign all
- Nothing will happen.

Origin of the issue:
====================
https://github.com/niklasvh/html2canvas/issues/2864

Solution:
=========
https://github.com/niklasvh/html2canvas/issues/2864#issuecomment-1111044861

opw-3660682

closes odoo/odoo#149903

X-original-commit: f11723f406f2af95090bdd13e9acb30c89014638
Signed-off-by: Jorge Pinna Puissant (jpp) <jpp@odoo.com>

addons/web_editor/static/lib/html2canvas.js

@@ -5603,15 +5603,19 @@
               return reject("No window assigned for iframe");
           }
           var documentClone = cloneWindow.document;
-          cloneWindow.onload = iframe.onload = function () {
-              cloneWindow.onload = iframe.onload = null;
-              var interval = setInterval(function () {
-                  if (documentClone.body.childNodes.length > 0 && documentClone.readyState === 'complete') {
-                      clearInterval(interval);
-                      resolve(iframe);
-                  }
-              }, 50);
-          };
+          /*
+            ODOO Patch for safari v 15.4+ in ios: onload is never called.
+            Issue link: https://github.com/niklasvh/html2canvas/issues/2864
+            Suggested change: https://github.com/niklasvh/html2canvas/issues/2864#issuecomment-1111044861
+            Version that solves the issue: not available yet
+          */
+          var interval = setInterval(function () {
+            if (documentClone.body.childNodes.length > 0 && (documentClone.readyState === 'complete' || documentClone.readyState === 'interactive')) {
+                clearInterval(interval);
+                resolve(iframe);
+            }
+          }, 50);
+          // End patch
       });
   };
   var ignoredStyleProperties = [