[FIX] web_editor: fix background postion option on columns

The background position snippet option makes a copy of the snippet to
use in the overlay, such that it is over all content and the user can
easily see the end result of dragging the background. However, an
oversight in the CSS didn't account for the possibility of the copied
element to have a relative max-width (% of its container), because of
this, the overlay for those items was incorrect. This commit fixes that
issue by overriding the max-width and max-height properties of the
copied item.

closes odoo/odoo#42106

X-original-commit: 3b8cadd579f3309640a938bcce39ac83ac7a6942
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web_editor/static/src/scss/wysiwyg_snippets.scss

@@ -938,6 +938,8 @@ body.editor_enable.editor_has_snippets {
                 bottom: 0 !important;
                 left: 0 !important;
                 transform: none !important;
+                max-width: unset !important;
+                max-height: unset !important;
             }
         }
     }