LibVideo/VP9: Implement MV reading & rectify MV storage issues

With this patch we are finally done with section 6.4.X of the spec :^)
The only parsing left to be done is 6.5.X, motion vector prediction.

Additionally, this patch fixes how MVs were being stored in the parser.
Originally, due to the spec naming two very different values very
similarly, these properties had totally wrong data types, but this has
now been rectified.

Userland/Libraries/LibVideo/VP9/MV.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2021, Hunter Salyer <thefalsehonesty@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "MV.h"
+
+namespace Video::VP9 {
+
+MV::MV(u32 row, u32 col)
+    : m_row(row)
+    , m_col(col)
+{
+}
+
+MV& MV::operator=(MV const& other)
+{
+    if (this == &other)
+        return *this;
+    m_row = other.row();
+    m_col = other.col();
+    return *this;
+}
+
+MV& MV::operator=(i32 value)
+{
+    m_row = value;
+    m_col = value;
+    return *this;
+}
+
+MV MV::operator+(MV const& other) const
+{
+    return MV(this->row() + other.row(), this->col() + other.col());
+}
+
+}