Fix crash on quit after removing a source (double-free)

MappingManager::removeSource() explicitly called source->~Source(). Because
the destructor is virtual, that ran the full destructor chain on an object
still owned by QSharedPointer (e.g. retained by the undo stack for undo), so
the object was destroyed a second time when the last shared pointer was
released — a double-free that crashed in ~QObject() on quit (and on undo).

It was most visible with Syphon sources, which carry extra state (an
Objective-C client and a queued signal connection), but the bug applied to
any removed source.

Drop the explicit destructor call and let QSharedPointer own the lifetime;
removing the manager's references is enough.

Author: aalex

src/core/MappingManager.cpp

@@ -119,12 +119,20 @@ bool MappingManager::removeSource(uid sourceId)
       removeLayer(it.key());
     }
 
-    // Remove source.
+    // Remove source. Its lifetime is managed by QSharedPointer: dropping the
+    // manager's references here is enough. The source may still be held by the
+    // undo stack (so it can be restored), and is freed once the last shared
+    // pointer is released.
+    //
+    // NOTE: do NOT call source->~Source() explicitly. Because ~Source() is
+    // virtual, that runs the full destructor chain on an object the shared
+    // pointers still own, so the object is destroyed a second time when the
+    // last reference is released — a double-free that crashes on quit/undo
+    // (notably with Syphon sources, which carry extra state).
     int idx = sourceVector.lastIndexOf(source);
     Q_ASSERT(idx != -1);
     sourceVector.remove(idx);
     sourceMap.remove(sourceId);
-    source->~Source(); // FIX ME: Explicit call of source destructor in order add Camera more than once
     return true;
   }
   else