Chunk processing

Added api to set chunk processing callbacks on depth and video streams. These enable bypassing the costly memcpy in stream_process().You can find a demo of the functionality in examples/chunkview.c that unpacks the frame on the fly and downsamples it to 320x240. Testing indicates that the overhead of getting a 320x240 frame, unpacked and converted to mm(passing a lookup table through dev->user_data) is roughly the same as getting the packed 640x480 frame. Thresholding could also be applied in the callback for even more gains etc.