[gnome-shell] blur-effect: Silence compiler warning

[Marge Bot <marge-bot src gnome org>]

commit e8f46f169c6598737cec2dbc20c7c278563f2f31
Author: Robert Mader <robert mader posteo de>
Date:   Tue Jan 26 21:25:47 2021 +0100

    blur-effect: Silence compiler warning
    
    Building with `debugoptimized` throws:
    `'paint_opacity' may be used uninitialized in this function`
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1596>

 src/shell-blur-effect.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/src/shell-blur-effect.c b/src/shell-blur-effect.c
index 7003b68f7c..16bf086de6 100644
--- a/src/shell-blur-effect.c
+++ b/src/shell-blur-effect.c
@@ -615,6 +615,10 @@ shell_blur_effect_paint_node (ClutterEffect           *effect,
         case SHELL_BLUR_MODE_BACKGROUND:
           paint_opacity = 255;
           break;
+
+        default:
+          g_assert_not_reached();
+          break;
         }
 
       if (needs_repaint (self, flags))