Add 4 to 3 scaling NEON optimization

Speed comparing with the one calling vpx_scaled_2d_neon()
  ~1.7 x in general
  ~2.8x for BILINEAR filter

BUG=webm:1419

Change-Id: I8f0a54c2013e61ea086033010f97c19ecf47c7c6
2 files changed