[IMP] web: makes signature scale to canvas size to avoid mobile/desktop inconsistency

In the sign app, if a signature is saved in mobile resolution, when the user tries to sign a document in desktop, the size that is shown is the mobile size, which is strange for the user.
This PR is an odoo change and it modifies the library method that adds the image to the canvas, resizing the image to the canvas size.

task-2479787

closes odoo/odoo#68160

Signed-off-by: Arnaud Joset <arj-odoo@users.noreply.github.com>

addons/web/__manifest__.py

@@ -390,6 +390,7 @@ This module provides the core of the Odoo Web Client.
             'web/static/src/js/libs/underscore.js',
             'web/static/src/js/libs/popper.js',
             'web/static/src/js/libs/zoomodoo.js',
+            'web/static/src/js/libs/jSignatureCustom.js',
             'web/static/src/js/chrome/keyboard_navigation_mixin.js',
             'web/static/src/js/core/abstract_service.js',
             'web/static/src/js/core/abstract_storage_service.js',

addons/web/static/src/js/libs/jSignatureCustom.js

@@ -0,0 +1,48 @@
+function _renderImageOnCanvas( data, formattype, rerendercallable ) {
+    'use strict'
+    // #1. Do NOT rely on this. No worky on IE 
+    //   (url max len + lack of base64 decoder + possibly other issues)
+    // #2. This does NOT affect what is captured as "signature" as far as vector data is 
+    // concerned. This is treated same as "signature line" - i.e. completely ignored
+    // the only time you see imported image data exported is if you export as image.
+
+    // we do NOT call rerendercallable here (unlike in other import plugins)
+    // because importing image does absolutely nothing to the underlying vector data storage
+    // This could be a way to "import" old signatures stored as images
+    // This could also be a way to import extra decor into signature area.
+
+    // Odoo override: 
+    // If a signature is saved in mobile resolution, when the user tries to sign a document in desktop, the size that is shown is the mobile size, which is strange for the user.
+    // This modifies the library method that adds the image to the canvas, resizing the image to the canvas size.
+    
+    var img = new Image()
+    // this = Canvas DOM elem. Not jQuery object. Not Canvas's parent div.
+    , c = this;
+
+    img.onload = function () {
+        var ctx = c.getContext("2d");
+        var oldShadowColor = ctx.shadowColor;
+        ctx.shadowColor = "transparent";
+        ctx.drawImage( 
+            img, 0, 0
+            , c.width
+            , c.height
+        );
+        ctx.shadowColor = oldShadowColor;
+    };
+
+    img.src = 'data:' + formattype + ',' + data;
+}
+
+// Supported image types
+const imageTypes = [
+    'image',
+    'image/png;base64',
+    'image/jpeg;base64',
+    'image/jpg;base64',
+];
+
+// Override
+for (const imageType of imageTypes) {
+    $.fn.jSignature("addPlugin", "import", imageType, _renderImageOnCanvas);
+}