video receive thread: fix race condition on displayMatrix_

Instances of std::shared_ptr are not thread-safe, even if the control block is.

Since AVBufer already has a refcounting system, use a unique_ptr instead,
and add a mutex to protect access during copy.

In practice, the mutex should almost never block since it's locked for
the minimum scope.

Change-Id: I5122e76dadb5da6c3738f8dc711698744b52315b