LibVideo/VP9: Fix rounding of components in the motion vector selection

The division in the `round_mv_...()` functions contained in the motion
vector selection process was done by bit shifting right. However, since
bit shifting negative values will truncate towards the negative end, it
was flooring instead of rounding.

This changes it to match the spec and rely on the compiler to simplify
down to a bit shift.

Userland/Libraries/LibVideo/VP9/Decoder.cpp

@@ -693,15 +693,15 @@ MotionVector Decoder::select_motion_vector(u8 plane, BlockContext const& block_c
     auto round_mv_comp_q2 = [&](MotionVector in) {
         // return (value < 0 ? value - 1 : value + 1) / 2
         return MotionVector {
-            (in.row() < 0 ? in.row() - 1 : in.row() + 1) >> 1,
-            (in.column() < 0 ? in.column() - 1 : in.column() + 1) >> 1
+            (in.row() < 0 ? in.row() - 1 : in.row() + 1) / 2,
+            (in.column() < 0 ? in.column() - 1 : in.column() + 1) / 2
         };
     };
     auto round_mv_comp_q4 = [&](MotionVector in) {
         // return (value < 0 ? value - 2 : value + 2) / 4
         return MotionVector {
-            (in.row() < 0 ? in.row() - 2 : in.row() + 2) >> 2,
-            (in.column() < 0 ? in.column() - 2 : in.column() + 2) >> 2
+            (in.row() < 0 ? in.row() - 2 : in.row() + 2) / 4,
+            (in.column() < 0 ? in.column() - 2 : in.column() + 2) / 4
         };
     };