Fixed|libdeng2: Crash when recreating canvas

When the audiences of the old canvas were copied to the new one,
the C++ default assignment operator was used. This meant a pointer
to the old audiences was copied, causing a crash when continuing
iteration through a deleted audience.

Todo for later: This could use further improvement, perhaps by
utilizing a QSet in Observers<>. If the observers set is destroyed
in the middle of iterating through a large number of observers,
there will likely still be a crash because the 'next' iterator of
the loop will become invalid (and not just point to the end of the set
as in this instance when there is a single observer).

Observers<>::Loop could also observe the set itself, although in
that case it would be necessary to derive a custom set class that
provides the necessary notifications to Loop.

doomsday/libdeng2/include/de/data/observers.h

@@ -113,9 +113,16 @@ namespace de
 {
     /**
      * Template for observer sets. The template type should be an interface
-     * implemented by all the observers.
+     * implemented by all the observers. @ingroup data
      *
-     * @ingroup data
+     * @todo This could use further improvement, perhaps by utilizing a QSet in
+     * Observers<>. If the observers set is destroyed in the middle of
+     * iterating through a large number of observers, there will likely still
+     * be a crash because the 'next' iterator of the loop will become invalid
+     * (and not just point to the end of the set as in this instance when there
+     * is a single observer). Observers<>::Loop could also observe the set
+     * itself, although in that case it would be necessary to derive a custom
+     * set class that provides the necessary notifications to Loop.
      */
     template <typename Type>
     class Observers
@@ -166,6 +173,10 @@ namespace de
     public:
         Observers() : _members(0) {}
 
+        Observers(Observers<Type> const &other) : _members(0) {
+            *this = other;
+        }
+
         virtual ~Observers() {
             clear();
         }
@@ -175,6 +186,22 @@ namespace de
             _members = 0;
         }
 
+        Observers<Type> &operator = (Observers<Type> const &other) {
+            // If _members already exists, the instance is retained
+            // in case someone is using it.
+            if(other._members) {
+                checkExists();
+                _members->clear();
+                DENG2_FOR_EACH_CONST(typename Members, i, *other._members) {
+                    add(*i);
+                }
+            }
+            else {
+                if(_members) _members->clear();
+            }
+            return *this;
+        }
+
         /// Add an observer into the set. The set does not receive
         /// ownership of the observer instance.
         void add(Type *observer) {

doomsday/libgui/src/canvas.cpp

@@ -278,7 +278,7 @@ void Canvas::notifyReady()
 
     DENG2_FOR_AUDIENCE(GLReady, i) i->canvasGLReady(*this);
 
-    // The Canvas instance might have been destroyed now.
+    // This Canvas instance might have been destroyed now.
 }
 
 void Canvas::paintGL()