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

test/resize_test.cc

@@ -114,7 +114,8 @@ class ResizeInternalTest : public ResizeTest {
 
 TEST_P(ResizeInternalTest, TestInternalResizeWorks) {
   ::libvpx_test::I420VideoSource video("hantro_collage_w352h288.yuv", 352, 288,
-                                       30, 1, 0, 5);
+                                       30, 1, 0, 6);
+  cfg_.rc_target_bitrate = 5000;
   ASSERT_NO_FATAL_FAILURE(RunLoop(&video));
 
   for (std::vector<FrameInfo>::iterator info = frame_info_list_.begin();