libvpx

Enable encode_breakout in real time encoding

In real time encoding, we enable encode_breakout to make encoding
fast. A speed feature "use_encode_breakout" is defined to set
encode_breakout thresholds for different speeds.

However, currently, static_thresh is an encoder option. The encode_
breakout can be turned off if user sets static_thresh=0 specifically.

The rtc set borg test result: (need to set --static_thresh=1)
speed -5, psnr loss -3.543%;
speed -4, psnr loss -2.358%;
speed -3, psnr loss -0.771%.

Encoding speed test:
speed -5, 11% - 60% speedup;
speed -4, 5.5% - 28% speedup;
speed -3, 0.8% - 7% speedup.

Change-Id: Icde592ffbe77eac7446f872a2e9eb2051733677b