* The areas allocated for BBitmaps were never deleted, even though the
app_server deleted its part when the memory got freed.
* This resulted in a constant memory increase if the application in question
would operate on many changing large bitmaps, like photos.
* Since the bitmaps are reference counted, we don't actually know when to delete
the areas, so that the app_server now notifies the client whenever that is
possible.
* This might fix#6824.
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