gtk3/menu: fix corner radius background

As the title says, fixes solid color behind gtk menu's rounded corner.

This change is inspired by https://github.com/vinceliuice/Mojave-gtk-theme/blob/4604817056f25b5a7e3aaf725ee45a4bd1f45dd6/src/sass/gtk/_common-3.0.scss#L118

GTK3's default theme also has a similar rule: https://gitlab.gnome.org/GNOME/gtk/-/blob/3.24.51/gtk/theme/Adwaita/_common.scss#L2202

Screenshots from gtk file dialog in Inkscape:

before	after

Related bug: https://bugs.kde.org/show_bug.cgi?id=437127

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As some additional context, gtk's default Adwaita theme just have menu border-radius disabled, but context-menu is still rounded:

• https://gitlab.gnome.org/GNOME/gtk/-/issues/1661
• https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/834

menu	context-menu