[PERF] web_editor: save original image size in element attributes

When computing the original size of the image, the original image is
loaded to make sure we get the right value.

This commit caches it into the `data-width` attribute of the image to
avoid loading the original image every time. This fixes the performance
penalty introduced in commit [1].

Because the attachment/original image could change between page reloads,
the `data-width` attribute is removed in `cleanForSave`.

[1]: 60e25f7

task-3847470

Author: agau-odoo

addons/web_editor/static/src/js/editor/snippets.options.js

@@ -6177,6 +6177,13 @@ const ImageHandlerOption = SnippetOptionWidget.extend({
         // This does not update the target.
         await this._applyOptions(false);
     },
+    /**
+     * @override
+     */
+    async cleanForSave() {
+        const img = this._getImg();
+        delete img.dataset.width;
+    },
 
     //--------------------------------------------------------------------------
     // Public
@@ -6413,6 +6420,10 @@ const ImageHandlerOption = SnippetOptionWidget.extend({
 
         if (update) {
             img.classList.add('o_modified_image_to_save');
+            if (!img.dataset.width) {
+                // Save image original width.
+                img.dataset.width = await this._getOriginalSize();
+            }
             this._setImageMimetype(img, mimetype);
             const loadedImg = await loadImage(dataURL, img);
             this._applyImage(loadedImg);
@@ -6556,8 +6567,13 @@ const ImageHandlerOption = SnippetOptionWidget.extend({
      */
     async _getOriginalSize() {
         const image = this._getImg();
-        const originalImage = await loadImage(this.originalSrc);
-        return Math.round(image.dataset.width) || originalImage.naturalWidth;
+        const storedOriginalWidth = Math.round(image.dataset.width);
+        if (storedOriginalWidth) {
+            return storedOriginalWidth;
+        } else {
+            const originalImage = await loadImage(this.originalSrc);
+            return originalImage.naturalWidth;
+        }
     },
     /**
      * @param {HTMLImageElement} img