1) The scan orders of rect txfm will be diagonal with shorter side
first
2) Neighbor is not updated because lv_map doesn't use the neighbor
3) Write a scan test to protect the scan order.
4) Performance on key frame is neutral. Will report the overall
performance by comment later.

BUG=aomedia:1369

Change-Id: I417392f2d9870e035532a2e1cc7cf71dc20ea219