A speedup feature that enforces the maximum number of reference frames
to evaluate for each arbitrary frame to be 6, as opposed to the maximum
syntax-allowed number of reference frames which is 7, through the
following rules:

(1) When all the possible reference frames are availble, we reduce the
    number of reference frames (7) by 1 by removing one reference
    frame;
(2) Always retain GOLDEN_FARME/ALTEF_FRAME;
(3) Check the earliest 2 remaining reference frames, and there are two
    options to remove one reference frame:
    (a) Remove the one with the lower quality factor; Otherwise if
    both frames have been coded at the same quality level, remove the
    earliest reference frame;
    (b) Always remove the earliest reference frame.
    Currently set option (a).

This patch has demonstrated an encoder speedup of ~8.5%. It is under
speed2 with "selective_ref_frame >= 2". (May be considered to move
to speed1 later once the coding performance impact i...