Peter M. Groen / openbr

Simplify SingleBuffer since controlling the total amount of frames being
processed works quite well.
Add a double-buffer scheme, which leads to a really marginal performance
improvement (but I think it's cool).

Rather than putting limits on the number of items in the buffers of individual
processing stages of a stream, put a limit on the total number of frames
actively being processed by the stream.

Add StreamTransform which represents applying a series of transforms to some
sequential input (e.g. a video). Handles reading video frames in and processing
them as they become available (rather than loading the entire video into memory
before processing it). The basic idea is a pipeline model, every child
transform of StreamTransform is run by a separate thread, (as is reading in
video frames). A thread waits for a frame as input, when input becomes
available performs its operation, and then places the frame on the following
stage's input buffer. Single threaded processing is not yet implemented.

Moved CompositeTransform to openbr_plugin.h
Add ContractTransform -- combines all input templates into a single template
Preliminary handling of time varying transforms in DistributeTemplateTransform,
the current behavior is not actually correct but will work for the single input
case.