[FIX] web_editor: fix nested snippets background filters

Before this commit, modifying the "bg_filter" of a snippet would also
affect its child snippets (for example, if a Carousel snippet had a
"bg_filter" that contained another snippet with its own "bg_filter").

To reproduce the bug:

- Drop a "Tabs" snippet on a page.
- Add a background image to the snippet and apply a background filter to
it.
- Drop a "Text" snippet in one of the tabs of the "Tabs" snippet.
- Add a background image to the "Text" snippet and apply a background
filter to it.
- Change the color of the background filter of the "Tabs" snippet.
- Bug: the background filter of the "Text" snippet is also changed.

task-3102275

Part-of: odoo/odoo#111681