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Adds a new method, IGBP::detachNextBuffer, that effectively does
dequeue + request + detach in a single call, but does not need to
know anything about the dequeued buffer, and will not block on
dequeue. This is mostly for the upcoming StreamSplitter to use in
its onBufferReleased callback.

Change-Id: Ie88a69de109003acebaa486a5b44c8a455726550