Add buffer-based descriptors

[lukesd]

Buffer-based descriptors allow the storage (inside the o.dot code) of the last N frames of some value. This will allow computing statistics features, e.g. mean, median, max, min, etc.

Implementation will use a circular buffer to minimize moving memory around.

Possible downside: the contents of the buffer will appear in any display of the o.dot frame, adding visual clutter.