shell: Add alpha to weston_curtain_create

Not all solid-colour views want to be opaque: sometimes we use them with
non-opaque alpha values in order to shade views underneath them.

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

tests/safe-signal-output-removal-test.c

@@ -80,7 +80,7 @@ static void
 output_create_view(struct test_output *t_output)
 {
        struct weston_curtain_params curtain_params = {
-	       .r = 0.5, .g = 0.5, .b = 0.5,
+	       .r = 0.5, .g = 0.5, .b = 0.5, .a = 1.0,
 	       .x = 0, .y = 0,
 	       .width = 320, .height = 240,
 	       .get_label = NULL,