Implements #162 Hardware compression

Use NVENC to encode H265/HEVC video on the GPU. It does both colour and lossless depth streaming. This involved a massive refactor of the encoding/decoding process, with the original JPG+PNG approach remaining as a backup. Currently there is a performance issue with frames being lost, and some instability when changing the second channel as the decoder gets out of sync with the encoder (with respect to needing an iframe).

Edit: Can now disable NVENC in config file. Also, frame drop issue seems to be true of software encoder also so needs further debugging work.