aom_cx_set_ref: Example showing setting a reference frame.

This code was pulled over from libvpx (89a8174 and 9aaa3c9
Make set_reference control API work in VP9 and VP10). The testing
script was added to insure it's working properly.

Change-Id: Ica311c169a1e51718503e23756c4c394818217dd
4 files changed