vpxenc.sh: Add basic vp9 multithread encode test.

- Change default real time speed to -6.
- Add vpxenc_vp9_webm_rt_multithread, which encodes
  niklas_1280_720_30.y4m with 2 to 4 threads using 2 to 4
  tile columns.

Change-Id: I4d86c3360aec67ae5d1ba82eb6e0f0be8068b5af
2 files changed