Spatial resamping of ZEROMV predictors

This patch allows coding frames using references of different
resolution, in ZEROMV mode. For compound prediction, either
reference may be scaled.

To test, I use the resize_test and enable WRITE_RECON_BUFFER
in vp9_onyxd_if.c. It's also useful to apply this patch to
test/i420_video_source.h:

  --- a/test/i420_video_source.h
  +++ b/test/i420_video_source.h
  @@ -93,6 +93,7 @@ class I420VideoSource : public VideoSource {

     virtual void FillFrame() {
       // Read a frame from input_file.
  +    if (frame_ != 3)
       if (fread(img_->img_data, raw_sz_, 1, input_file_) == 0) {
         limit_ = frame_;
       }

This forces the frame that the resolution changes on to be coded
with no motion, only scaling, and improves the quality of the
result.

Change-Id: I1ee75d19a437ff801192f767fd02a36bcbd1d496

vp9/common/vp9_convolve.h

@@ -33,11 +33,8 @@ void vp9_convolve_avg(const uint8_t *src, int src_stride,
                       int w, int h);
 
 struct subpix_fn_table {
-  convolve_fn_t predict[2][2][2];  // horiz, vert, avg
   const int16_t (*filter_x)[8];
   const int16_t (*filter_y)[8];
-  int x_step_q4;
-  int y_step_q4;
 };
 
 #endif  // VP9_COMMON_CONVOLVE_H_