Add rounded windows with picom

picom.nix

@@ -3,5 +3,12 @@
 {
   services.picom = {
     enable = true;
+    settings = {
+      corner-radius = 12;
+      round-borders = 1;
+      rounded-borders-exclude = [
+        # TODO: figure out how to exclude polybar, smart gaps
+      ];
+    };
   };
 }