vpStream: Video stream object.
Description
A vpStream object is an R container for an OpenCV camera stream. vpStream 
objects can be generated by the readStream function in the 
videoplayR library.
Format
A vpStream object contains one field:
- "dim": the dimensions in pixels (width x height) of the video stream.
Details
A vpStream object is not a persistent object, but a pointer to a C++ 
object that cannot be saved for reuse in a different session.