Resources: Clear all materials when a new game is loaded

It is presently necessary to clear all materials from the resource
system when loading a new game.

Todo for later: It should only be necessary to clear the "runtime"
(i.e., game) materials rather than all of them.

doomsday/client/src/dd_main.cpp

@@ -1413,6 +1413,9 @@ bool App_ChangeGame(Game &game, bool allowReload)
     FI_Shutdown();
     titleFinale = 0; // If the title finale was in progress it isn't now.
 
+    /// @todo The entire material collection should not be destroyed during a reload.
+    App_ResourceSystem().clearAllMaterialSchemes();
+
     if(!game.isNull())
     {
         LOG_VERBOSE("Selecting game '%s'...") << Str_Text(game.identityKey());