Fix to source scaling for dynamic_resize.

The fast scaling for 1 pass mode was being used only on the
first frame after resizing event (because resize_scale_num/den
is set to 1 and only changed for first frame following resize event).

Change-Id: I723b63e21823eb858f25f5662d2bbe4f1842e61f

vp9/encoder/vp9_encoder.c

diff --git a/vp9/encoder/vp9_encoder.c b/vp9/encoder/vp9_encoder.c
index 43d459e..d5ac3e3 100644
--- a/vp9/encoder/vp9_encoder.c
+++ b/vp9/encoder/vp9_encoder.c
@@ -3147,8 +3147,8 @@
   if (cpi->oxcf.pass == 0 &&
       cpi->oxcf.rc_mode == VPX_CBR &&
       cpi->oxcf.resize_mode == RESIZE_DYNAMIC &&
-      cpi->resize_scale_num == 1 &&
-      cpi->resize_scale_den == 2) {
+      cpi->un_scaled_source->y_width == (cm->width << 1) &&
+      cpi->un_scaled_source->y_height == (cm->height << 1)) {
     cpi->Source = vp9_scale_if_required_fast(cm,
                                              cpi->un_scaled_source,
                                              &cpi->scaled_source);