Reduced time-desync of cameras
 - seperated video read() into grab() and capture()
 - grab gets the frame with no extra processing
 - retrieve decodes frame
 - seperating operations allows us to grab both left and right frames with no retrieving/decoding happening to delay the second image
 - http://docs.opencv.org/modules/highgui/doc/reading_and_writing_images_and_video.html#videocapture-grab