screenshot: Disable screencasts when locked

It is currently not possible to bring up the screenshot UI while
locked, but that is about to change.

We still don't want screencasts in that case, because they are
much easier to abuse for filling up someone else's disk.

That restriction is enforced by inhibiting remote access in the
backend, so trying to create a screencast session will fail anyway.
Still, not offering an action that is unavailable is better than
having it fail silently, so do exactly that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2439>

data/theme/gnome-shell-sass/widgets/_screenshot.scss

@@ -98,6 +98,7 @@ $screenshot_ui_button_red: $error_color;
   &:hover, &:focus { background-color: lighten($osd_bg_color, 10%);}
   &:active { background-color: lighten($active_bg_color,5%);}
   &:checked { background-color: white;color: black;}
+  &:insensitive { color: transparentize($osd_fg_color, 0.5);}
 
   border-radius: $modal_radius - $screenshot_ui_shot_cast_spacing;