Forward-coded KEY_FRAME, served as a backward reference frame, is
coded as intra-only. The show_existing_frame to show the buffered
forward KEY_FRAME needs to reset the frame context as well as reset
the reference frame buffer.

One binary symbol, namely reset_decoder_state, is added to the frame
header. Whenever a frame is a show_existing_frame, it reads out this
binary symbol value from the bitstream. When this binary symbol is
1, it indicates that the existing frame to show shall be an intra
coded frame and will serve as a KEY_FRAME. The frame context is set
to default and the reference buffer is updated the same way as a
normal KEY_FRAME.

Change-Id: I8b641220689459a104d2f5a03bbdb6820af8f990