LibVideo/VP9: Use a bitwise enum for motion vector joint selection

The motion vector joints enum is set up so that the first bit indicates
that a vector should have a non-zero value in the column, and the
second bit indicates a non-zero value for the row. Taking advantage of
this makes the code a bit more legible.

Userland/Libraries/LibVideo/VP9/Enums.h

@@ -113,10 +113,9 @@ enum class PredictionMode : u8 {
 };
 
 enum MvJoint : u8 {
-    MvJointZero = 0,
-    MvJointHnzvz = 1,
-    MvJointHzvnz = 2,
-    MvJointHnzvnz = 3,
+    MotionVectorAllZero = 0,
+    MotionVectorNonZeroColumn = 1,
+    MotionVectorNonZeroRow = 2,
 };
 
 enum MvClass : u8 {

Userland/Libraries/LibVideo/VP9/LookupTables.h

@@ -142,9 +142,9 @@ static constexpr int interp_filter_tree[4] = {
     -EightTapSmooth, -EightTapSharp
 };
 static constexpr int mv_joint_tree[6] = {
-    -MvJointZero, 2,
-    -MvJointHnzvz, 4,
-    -MvJointHzvnz, -MvJointHnzvnz
+    -MotionVectorAllZero, 2,
+    -MotionVectorNonZeroColumn, 4,
+    -MotionVectorNonZeroRow, -(MotionVectorNonZeroColumn | MotionVectorNonZeroRow)
 };
 static constexpr int mv_class_tree[20] = {
     -MvClass0, 2,

Userland/Libraries/LibVideo/VP9/Parser.cpp

@@ -1276,14 +1276,14 @@ static bool should_use_high_precision_motion_vector(MotionVector const& delta_ve
 DecoderErrorOr<MotionVector> Parser::read_motion_vector(BlockContext const& block_context, BlockMotionVectorCandidates const& candidates, ReferenceIndex reference_index)
 {
     m_use_hp = block_context.frame_context.high_precision_motion_vectors_allowed && should_use_high_precision_motion_vector(candidates[reference_index].best_vector);
-    MotionVector diff_mv;
-    auto mv_joint = TRY_READ(TreeParser::parse_motion_vector_joint(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter));
-    if (mv_joint == MvJointHzvnz || mv_joint == MvJointHnzvnz)
-        diff_mv.set_row(TRY(read_single_motion_vector_component(0)));
-    if (mv_joint == MvJointHnzvz || mv_joint == MvJointHnzvnz)
-        diff_mv.set_column(TRY(read_single_motion_vector_component(1)));
-
-    return candidates[reference_index].best_vector + diff_mv;
+    MotionVector delta_vector;
+    auto joint = TRY_READ(TreeParser::parse_motion_vector_joint(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter));
+    if ((joint & MotionVectorNonZeroRow) != 0)
+        delta_vector.set_row(TRY(read_single_motion_vector_component(0)));
+    if ((joint & MotionVectorNonZeroColumn) != 0)
+        delta_vector.set_column(TRY(read_single_motion_vector_component(1)));
+
+    return candidates[reference_index].best_vector + delta_vector;
 }
 
 // read_mv_component( comp ) in the spec.