picom: add window shadows

modules/picom/default.nix

@@ -3,6 +3,11 @@
 {
   services.picom = {
     enable = true;
+    shadow = true;
+    shadowOffsets = [
+      (-15)
+      (-15)
+    ];
     settings = {
       backend = "glx"; # Fixes black backdrop on some rounded corners
       corner-radius = 12;