Change mv projection to signed rounding

The numerator in the mv projection can be negative, e.g. cur_to_bwd
or cur_to_alt2, since either bwdref or altref2 can be a forward
predictive reference, whereas the denominator always stays positive.
The rounding inside mv projection hence should use signed operation.

Change-Id: I42a105835754a002dd31fcfa7c845e4c105ec54f

av1/common/mvref_common.c

diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index d92585a..3f1d2c8 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -1468,9 +1468,9 @@
 // altogether.
 static void get_mv_projection(MV *output, MV ref, int num, int den) {
   output->row =
-      (int16_t)(ROUND_POWER_OF_TWO(ref.row * num * div_mult[den], 14));
+      (int16_t)(ROUND_POWER_OF_TWO_SIGNED(ref.row * num * div_mult[den], 14));
   output->col =
-      (int16_t)(ROUND_POWER_OF_TWO(ref.col * num * div_mult[den], 14));
+      (int16_t)(ROUND_POWER_OF_TWO_SIGNED(ref.col * num * div_mult[den], 14));
 }
 
 #define MAX_OFFSET_WIDTH 64