Output: don't configure uninitialized layer surfaces

It is possible for a layer surface to notably delay its initial commit;
for example shotman[1] creates two layer surfaces and uses one of them
to get enough information for a screenshot and initializing the other.
River could also have sent a configure before initial commit if two
clients raced against each other.

Fixes https://codeberg.org/river/river/issues/1123

[1]:https://sr.ht/~whynothugo/shotman/

(cherry picked from commit 93863b132eb7a32e296d5f224181b04e161b1c58)

river/Output.zig

@@ -367,6 +367,8 @@ fn sendLayerConfigures(
             if (@as(?*SceneNodeData, @ptrFromInt(node.data))) |node_data| {
                 const layer_surface = node_data.data.layer_surface;
 
+                if (!layer_surface.wlr_layer_surface.initialized) continue;
+
                 const exclusive = layer_surface.wlr_layer_surface.current.exclusive_zone > 0;
                 if (exclusive != (mode == .exclusive)) {
                     continue;