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/Software/SWBoundingBox.h

@@ -0,0 +1,43 @@
+// Copyright 2021 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "Common/CommonTypes.h"
+
+#include "VideoCommon/BoundingBox.h"
+
+namespace BBoxManager
+{
+// Indicates a coordinate of the bounding box.
+enum class Coordinate
+{
+  Left,    // The X coordinate of the left side of the bounding box.
+  Right,   // The X coordinate of the right side of the bounding box.
+  Top,     // The Y coordinate of the top of the bounding box.
+  Bottom,  // The Y coordinate of the bottom of the bounding box.
+};
+
+// Gets a particular coordinate for the bounding box.
+u16 GetCoordinate(Coordinate coordinate);
+
+// Sets a particular coordinate for the bounding box.
+void SetCoordinate(Coordinate coordinate, u16 value);
+
+// Updates all bounding box coordinates.
+void Update(u16 left, u16 right, u16 top, u16 bottom);
+}  // namespace BBoxManager
+
+namespace SW
+{
+class SWBoundingBox final : public BoundingBox
+{
+public:
+  bool Initialize() override { return true; }
+
+protected:
+  std::vector<BBoxType> Read(u32 index, u32 length) override;
+  void Write(u32 index, const std::vector<BBoxType>& values) override;
+};
+
+}  // namespace SW