Disable specialcase for last frames if the sequence contains ARFs.

firstpass.c contains some rate adjustment code that assures that the
last few frames in a sequence abide by rate limits. If the second-to-
last group of frames contains an alt-ref frame (ARF), the last golden
frame (GF) is zero bytes, and we will thus spend a ridiculously high
number of bits on regular P-frames trying to hit the target rate. This
does slightly enhance the quality of these last few frames, but has
no perceptual value (other than hitting the target rate).

Disabling this code means we consistently (slightly) undershoot the
target rate and consequently do worse on the last few frames of a
clip, which is particularly noticeable for small clips. The quality-
per-bitrate is generally better, ~0.2% better overall on derf-set,
especially on clips such as garden, tennis, foreman at low bitrates.
Has a negative effect on hallmonitor at high bitrates.

Change-Id: I1d63452fef5fee4a0ad2fb2e9af4c9f2e0d86d23