added threading to v4l2 capture
added env var 'VEEJAY_V4L2_ALL_IN_THREAD' (set to 1 to move all image processing into thread and memcpy the captured frame to consumer)
added env var 'VEEJAY_V4L2_MAX_RETRIES' (try before giving up)
added env var 'VEEJAY_V4L2_CAPTURE_METHOD' (set to 0 to use read/write interface)
poll before pull frame
changed default to threaded v4l2 capture