Add try_level_down()
Add try_level_down()

This function computes the overall (i.e. self and neighbors') cost
difference caused by downgrading a coefficient by one level at
a specific location

Change-Id: I1b7b6acfe06ed06b9a2ff48b5bb11527646d1aa8

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom