Updates for resolution adaptation.
1) added support for two additional modes: 
    -3/4 spatial down-sampling
    -2/3 frame rate reduction
2) updated unittest and added a few more tests
3) some code refactoring
Review URL: https://webrtc-codereview.appspot.com/429005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@1854 4adac7df-926f-26a2-2b94-8c16560cd09d
5 files changed