Use stable sort with PVQ.

Cherry-pick Daala 85433214
 Fully order the pvq search candidates
For portable and stable sorting, break ties.
Large differences in output were observed between AWCY and an OS X
 machine because of the platform qsort implementation.

Change-Id: I294dd2e167c1e0464c7f61f32d60ab478341446e
1 file changed