Consider 4 neighbors in try_level_down()
Consider 4 neighbors in try_level_down()

This will let the coding performance drop by 0.1%,
but it will speed up encoder by 40%

The change is under the flag FAST_OPTIMIZE_TXB

Change-Id: I07dea24e818123e27c4347189aaec59deb42903c

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