Root: keep all fullscreen views the correct size

Currently we may resize fullscreen views when they become visible/not
visible when switching tags even if their fullscreen state remains
constant. This is suboptimal, and as it turns out also much more complex
to implement.

river/Output.zig

@@ -121,9 +121,6 @@ pending: struct {
     ///
     /// This includes both floating/fullscreen views and those arranged in the layout.
     wm_stack: wl.list.Head(View, .pending_wm_stack_link),
-    /// The view to be made fullscreen, if any.
-    /// This state should only be read/written inside Root.applyPending()
-    fullscreen: ?*View = null,
 },
 
 /// The state most recently sent to the layout generator and clients.