Make Bayer functions with same parameters as RGB conversions allowing consistant usage and testing.  Functions are implemented with wrappers to single function for 4 fourccs.  New I420ToBayer functions implemented similar to 2 step I420ToRGB565 etc.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/367002

git-svn-id: http://libyuv.googlecode.com/svn/trunk@150 16f28f9a-4ce2-e073-06de-1de4eb20be90