Frank Bossen authored Jul 25, 2018 and Yushin Cho committed Jul 25, 2018

* Define reference frame buffer array

A reference frame buffer with 8 slots is defined. The buffer is
updated after encoding each frame according to the value of
refresh_frame_flags. The value of refresh_frame_flags is also
set and properly encoded.

* Fix test function

Also move update of frame buffer to a separate function

* Use named constants instead of magic numbers