libvpx

Enable reduced set of intra modes in rtc coding

This commit enables the use of DC, vertical, and horizontal intra
prediction mode in rtc non-RD mode decision. When the best cost value
of inter modes is above a given threshold, the encoder runs the
above three intra modes and selects the one that has minimum
prediction residual in terms of SAD.

This together with recent changes on non-RD mode decision and coding
control improves compression performance of speed -6 by
derf  91%
yt    61%
hd    46%
stdhd 52%

In terms of encoding speed, it is about 3 times faster than speed -5.

Change-Id: I6b483bfd0307e6482bb22a6676ae4e25a52b1310