Frames can be dropped at the beginning and redundant frames at the end

[minseungchoi]

Because writer thread starts after cam acquisition, there's a gap of time when frames could be dropped.

Also, because we insert to image_queue from self.frame, we could be writing redundant frames when no frames are grabbed.

[minseungchoi]

Addressed by the previous few commits. Changed order of recording start. We prevent redundant frame writing.