Prior to this commit, when using the "Slide Hover" reveal effect on the
footer, Safari would glitch it scrolls and could result in unreadable
content.

This commit fixes this by a weird hack that seems to work. Adding an
element with a background-image and a background-attachment set to
fixed seems to resolve the issue.

task-3302302

closes odoo/odoo#125229

X-original-commit: a6307448
Signed-off-by: Dieleman Guillaume (gdi) <gdi@odoo.com>