[FIX] web_editor: show background image options after save

Steps to reproduce the bug:
- Add a cover snippet on the website.
- Change the background image with one of your own.
- Save and edit again.
=> Options such as "Filter", "Width" and "Quality" do not appear
anymore.

The problem is that the `src` attribute of the background image is not
correctly updated when initializing the image. This is because the
target used to recover the URL of the background image is the snippet
and not the image itself. This problem is resolved by ensuring that the
argument of the function `getBgImageURL` is the background image. Now
that the `src` attribute is correctly updated, the `computeVisibility`
function of the the `BackgroundOptimize` option works properly and the
options "Filter", "Width" and "Quality" are displayed as wanted.

Note that now, the dataset of the target is filtered with a "white list"
(BACKGROUND_IMAGE_ATTRIBUTES) before being copied in the dataset of
`this.img`. Indeed, if the parallax is set to "None" for example, we do
not want data attributes such as `data-snippet` to be copied in the
dataset of `this.img`.

task-3287330

Part-of: odoo/odoo#119596