libdeng2: Clear log buffer before unloading any library

It is possible that the library has created log entries that are still
in the log buffer. Previously this was done in 'engine', but now since
libdeng2 is in control of all plugin loading and unloading, it can
clear the log buffer in one central place (just before closing the
library).

doomsday/engine/src/dd_main.cpp

@@ -1475,9 +1475,6 @@ bool DD_ChangeGame(de::Game& game, bool allowReload = false)
         }
     )
 
-    // Remove all entries; some may have been created by the game plugin (if it used libdeng2 C++ API).
-    LogBuffer_Clear();
-
     Library_ReleaseGames();
 
     char buf[256];

doomsday/engine/src/dd_plugin.c

@@ -146,9 +146,6 @@ void Plug_UnloadAll(void)
 {
     int i;
 
-    // Remove all entries; some may have been created by the plugins.
-    LogBuffer_Clear();
-
     for(i = 0; i < MAX_PLUGS && hInstPlug[i]; ++i)
     {
         unloadPlugin(&hInstPlug[i]);

doomsday/libdeng2/include/de/core/logbuffer.h

@@ -73,6 +73,10 @@ class DENG2_PUBLIC LogBuffer : public QObject, public Lockable, DENG2_OBSERVES(F
      */
     void add(LogEntry *entry);
 
+    /**
+     * Clears the buffer by deleting all entries from memory. However, they are
+     * first flushed, so the no entries are lost.
+     */
     void clear();
 
     /**

doomsday/libdeng2/src/core/library.cpp

@@ -20,6 +20,7 @@
 #include "de/libdeng2.h"
 #include "de/Library"
 #include "de/Log"
+#include "de/LogBuffer"
 
 #if defined(UNIX) && defined(DENG2_QT_4_7_OR_NEWER) && !defined(DENG2_QT_4_8_OR_NEWER)
 #  define DENG2_USE_DLOPEN
@@ -117,6 +118,10 @@ Library::~Library()
             DENG2_SYMBOL(deng_ShutdownPlugin)();
         }
 
+        // The log buffer may contain log entries built by the library; those
+        // entries contain pointers to functions that are about to disappear.
+        LogBuffer::appBuffer().clear();
+
 #ifndef DENG2_USE_DLOPEN
         d->library->unload();
         delete d->library;