wiener_test: Dedup compute_stats_win* funcs.
wiener_test: Dedup compute_stats_win* funcs.

Using vectors instead of arrays to avoid variable-length-arrays.

BUG=aomedia:2044

Change-Id: Iddf2a0b56759d1a74adfddc34034063c04a4f7fc

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