ext-tx: use raster scan order for identity transform
ext-tx: use raster scan order for identity transform

coding gain of ext-tx:
screen_content 12.73% -> 13.05%

Change-Id: I5fc8cf0db84c3e56dd3cb7675e1d81c9c575bc57

mbebenita/aom,mbebenita/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,mbebenita/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom