Store result on conv_params->buf when no rounding

We need uint16_t buf for storing no-rounding prediction.
Add uint16_t buf in conv_params for that.
This CL let us avoid changing interface of convolve functions.

Change-Id: I079fad911327f40ffb98e17c73e7638b1719c975
diff --git a/av1/common/convolve.h b/av1/common/convolve.h
index 7b4e141..e62bcea 100644
--- a/av1/common/convolve.h
+++ b/av1/common/convolve.h
@@ -26,6 +26,8 @@
 typedef struct ConvolveParams {
   int ref;
   CONVOLVE_OPT round;
+  uint16_t *dst;
+  int dst_stride;
 } ConvolveParams;
 
 static INLINE ConvolveParams get_conv_params(int ref) {