Add support for multi-threaded execution of time invariant stages of a stream
Fix a crash in Show2 transform

Send a deep copy of the current image to the GUI thread, copy must be called
explicitly or a shallow copy will be done, and subsequent modifications to the
buffer will cause crashes when the GUI thread tries to draw it.
Merge pull request #32 from biometrics/stream_processing

Updated stream multi-threading scheme to allow parallel processing of frames by time invariant transforms