transactions: handle preemption properly

when a transaction interrupts an ongoing transaction, we must be careful
to handle the configures properly. This commit adds a new member to view
so that we can store the dimensions sent with the last configure in
order to determine if the preempting transaction should override the
ongoing configure or not.

Additionally, some views do not ack a configure if they already have the
requested dimensions. This can happen if a pending configure setting
alternative dimensions is overridden, so in this case we do not wait for
an ack before committing the transaction.

river/XdgToplevel.zig

@@ -63,11 +63,11 @@ pub fn init(self: *Self, view: *View, wlr_xdg_surface: *c.wlr_xdg_surface) void
     c.wl_signal_add(&self.wlr_xdg_surface.events.unmap, &self.listen_unmap);
 }
 
-pub fn configure(self: Self, pending_box: Box) void {
+pub fn configure(self: Self, box: Box) void {
     self.view.pending_serial = c.wlr_xdg_toplevel_set_size(
         self.wlr_xdg_surface,
-        pending_box.width,
-        pending_box.height,
+        box.width,
+        box.height,
     );
 }