theme: Restore flat bg-img on backdrop check/radio

In GTK3, check and radio nodes in the :backdrop state had their backdrop
image removed/flattened, like 'normal' buttons. In GTK4 as of now, this
does not happen. It is because _common.scss simply does not call check()
with the :backdrop state, to generate CSS to flatten the bg-image when
backdropped. This makes that happen, so indicators in backdrop go flat.

gtk/theme/Default/_common.scss

@@ -2570,6 +2570,7 @@ radio {
     @each $state, $t in ("", "normal"),
                         (":hover", "hover"),
                         (":active", "active"),
+                        (":backdrop", "backdrop"),
                         (":disabled", "insensitive") {
       &#{$state} {
         @include check($t, $_c);
@@ -2584,6 +2585,7 @@ radio {
         @each $state, $t in ("", "normal"),
                             (":hover", "hover"),
                             (":active", "active"),
+                            (":backdrop", "backdrop"),
                             (":disabled", "insensitive") {
           &#{$state} {
             @include check($t, $checkradio_bg_color, $checkradio_fg_color, $checked: true);