View: fix unmap/destroy control flow

Currently the view destruction sequence is started as soon as a view
is unmapped. However, this is incorrect as a client may map the view
again instead of destroying it.

Instead, only start the view destruction sequence when the underlying
xdg toplevel or xwayland surface is destroyed.

river/command/focus_view.zig

@@ -72,5 +72,5 @@ pub fn focusView(
 }
 
 fn filter(view: *View, filter_tags: u32) bool {
-    return !view.destroying and view.pending.tags & filter_tags != 0;
+    return view.surface != null and view.pending.tags & filter_tags != 0;
 }

river/command/swap.zig

@@ -80,6 +80,6 @@ pub fn swap(
 }
 
 fn filter(view: *View, filter_tags: u32) bool {
-    return !view.destroying and !view.pending.float and
+    return view.surface != null and !view.pending.float and
         !view.pending.fullscreen and view.pending.tags & filter_tags != 0;
 }

river/command/zoom.zig

@@ -61,6 +61,6 @@ pub fn zoom(
 }
 
 fn filter(view: *View, filter_tags: u32) bool {
-    return !view.destroying and !view.pending.float and
+    return view.surface != null and !view.pending.float and
         !view.pending.fullscreen and view.pending.tags & filter_tags != 0;
 }