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.cpp

@@ -0,0 +1,66 @@
+// Copyright 2021 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "VideoBackends/Software/SWBoundingBox.h"
+
+#include <algorithm>
+#include <array>
+
+#include "Common/CommonTypes.h"
+
+namespace BBoxManager
+{
+namespace
+{
+// Current bounding box coordinates.
+std::array<u16, 4> s_coordinates{};
+}  // Anonymous namespace
+
+u16 GetCoordinate(Coordinate coordinate)
+{
+  return s_coordinates[static_cast<u32>(coordinate)];
+}
+
+void SetCoordinate(Coordinate coordinate, u16 value)
+{
+  s_coordinates[static_cast<u32>(coordinate)] = value;
+}
+
+void Update(u16 left, u16 right, u16 top, u16 bottom)
+{
+  const u16 new_left = std::min(left, GetCoordinate(Coordinate::Left));
+  const u16 new_right = std::max(right, GetCoordinate(Coordinate::Right));
+  const u16 new_top = std::min(top, GetCoordinate(Coordinate::Top));
+  const u16 new_bottom = std::max(bottom, GetCoordinate(Coordinate::Bottom));
+
+  SetCoordinate(Coordinate::Left, new_left);
+  SetCoordinate(Coordinate::Right, new_right);
+  SetCoordinate(Coordinate::Top, new_top);
+  SetCoordinate(Coordinate::Bottom, new_bottom);
+}
+
+}  // namespace BBoxManager
+
+namespace SW
+{
+std::vector<BBoxType> SWBoundingBox::Read(u32 index, u32 length)
+{
+  std::vector<BBoxType> values(length);
+
+  for (u32 i = 0; i < length; i++)
+  {
+    values[i] = BBoxManager::GetCoordinate(static_cast<BBoxManager::Coordinate>(index + i));
+  }
+
+  return values;
+}
+
+void SWBoundingBox::Write(u32 index, const std::vector<BBoxType>& values)
+{
+  for (size_t i = 0; i < values.size(); i++)
+  {
+    BBoxManager::SetCoordinate(static_cast<BBoxManager::Coordinate>(index + i), values[i]);
+  }
+}
+
+}  // namespace SW