LibVideo: Put motion vector or reference frame pairs in a struct

Since these two types are often passed around as a pair, it's easier to
handle them with a simple pair struct, at least for now. Once things
are fully being passed around as parameters wherever possible, it may
be good to change this type for something more generalized.

Userland/Libraries/LibVideo/VP9/Context.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Hunter Salyer <thefalsehonesty@gmail.com>
+ * Copyright (c) 2022, Gregory Bertilson <zaggy1024@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Enums.h"
+#include "MotionVector.h"
+
+namespace Video::VP9 {
+
+template<typename T>
+struct Pair {
+    T a;
+    T b;
+
+    T& operator[](size_t index)
+    {
+        if (index == 0)
+            return a;
+        if (index == 1)
+            return b;
+        VERIFY_NOT_REACHED();
+    }
+};
+
+typedef Pair<ReferenceFrameType> ReferenceFramePair;
+typedef Pair<MotionVector> MotionVectorPair;
+
+}