allow dimensions as low as 1 pixel

remove warning comment in vpxenc.c: in case of 1x1 picture,
detect_bytes will be equal to '3' and we'll fall back to
RAW_TYPE.
fix read_frame() by tracking the pre-read buffer length
in the struct detect

Change-Id: If1ed86ee5260dcdbc8f9d10da6cbb84a4cc2f151