shell: Make input capture optional for curtains

desktop-shell's focus surfaces want to reuse this, but they don't want
to capture the input, instead allowing it to fall through.

Signed-off-by: Daniel Stone <daniels@collabora.com>

kiosk-shell/kiosk-shell.c

@@ -508,6 +508,8 @@ kiosk_shell_output_recreate_background(struct kiosk_shell_output *shoutput)
 	curtain_params.width = output->width;
 	curtain_params.height = output->height;
 
+	curtain_params.capture_input = true;
+
 	curtain_params.get_label = kiosk_shell_background_surface_get_label;
 	curtain_params.surface_committed = NULL;
 	curtain_params.surface_private = NULL;