VideoCommon: Abstract bounding box

This moves much of the duplicated bounding box code into VideoCommon,
leaving only the specific buffer implementations in each backend.

Source/Core/VideoBackends/Null/NullBoundingBox.h

@@ -0,0 +1,25 @@
+// Copyright 2021 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "Common/CommonTypes.h"
+
+#include "VideoCommon/BoundingBox.h"
+
+namespace Null
+{
+class NullBoundingBox final : public BoundingBox
+{
+public:
+  bool Initialize() override { return true; }
+
+protected:
+  std::vector<BBoxType> Read(u32 index, u32 length) override
+  {
+    return std::vector<BBoxType>(length);
+  }
+  void Write(u32 index, const std::vector<BBoxType>& values) override {}
+};
+
+}  // namespace Null