output: remove wl_output globals for mirrored displays

ref #6387

src/protocols/core/Output.cpp

@@ -105,3 +105,7 @@ void CWLOutputProtocol::remove() {
     defunct = true;
     removeGlobal();
 }
+
+bool CWLOutputProtocol::isDefunct() {
+    return defunct;
+}

src/protocols/core/Output.hpp

@@ -40,6 +40,7 @@ class CWLOutputProtocol : public IWaylandProtocol {
 
     // will mark the protocol for removal, will be removed when no. of bound outputs is 0 (or when overwritten by a new global)
     void remove();
+    bool isDefunct(); // true if above was called
 
   private:
     void destroyResource(CWLOutputResource* resource);