libvpx

Rework mode search threshold update for RTC coding mode

In RTC coding mode, the alternate reference frame modes and compound
inter prediction modes are disabled. This commit reworks the
related mode search threshold update process to skip interacting
with these coding modes. It provides about 1.5% speed-up for speed
-6 on average.

vidyo1
16551 b/f, 40.451 dB, 6261 ms -> 16550 b/f, 40.459 dB, 6190 ms

nik720p
33316 b/f, 38.795 dB, 6335 ms -> 33310 b/f, 38.798 dB, 6237 ms

mmmoving
33265 b/f, 41.055 dB, 7176 ms -> 33267 b/f, 41.064 dB, 7084 ms

dark720
33329 b/f, 39.729 dB, 11235 ms -> 33331 b/f, 39.733 dB, 10731 ms

Change-Id: If2a4090a371cd28f579be219c013b972d7d9b97f