canvas vg: drop vg cache buffers if object were destroyed.
Previous vg didn't take care of cached buffers which were
allocated in it's lifetime because the cache buffers are managed
by its own cache buffer mgr, it has a limitation count of buffers also
buffers can be cleared when engine is shutdown.
This behavior is actually working properly but not well optimized
since it lost a chance to clear grown buffers.
Now vg do clear used buffers when object is invalidated.