For consistency with upsampling filter and dr_predictor_*
functions, filter (w + h) pixels, instead of num_available.

BD-rate change, 4 key frames:

           PSNR    SSIM
lowres:  -0.010  +0.004
midres:  -0.004  -0.004
hdres:   -0.014  -0.021

BUG=aomedia:1314

Change-Id: I6363e9288fa64cbf90b32887165a33dce686fc00