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 {