layer-shell: handle commits before map

A client is free to change its mind and request a different
size/anchor/etc after recieving the initial configure but before
attaching and committing the first buffer. This means that we should
respond to such a situation with a new configure.

mako has been observed doing this in the wild for example.

river/Root.zig

@@ -490,7 +490,7 @@ fn applyOutputConfig(self: *Self, config: *wlr.OutputConfigurationV1) bool {
         // Arrange layers to adjust the usable_box
         // We dont need to call arrangeViews() since arrangeLayers() will call
         // it for us because the usable_box changed
-        output.arrangeLayers();
+        output.arrangeLayers(.mapped);
         self.startTransaction();
     }