add A2 stub

src/a2.cpp

@@ -0,0 +1,18 @@
+#include "a2.h"
+
+namespace freling {
+
+std::vector<BoundingBox> A2(const std::vector<BoundingBox>& regions) {
+    std::vector<BoundingBox> updated_boxes(regions.size());
+    for (int i = 0, size = regions.size(); i < size; ++i) {
+        auto& box = updated_boxes[i];
+        const auto& region = regions[i];
+        box.x = region.x + 2;
+        box.y = region.y + 2;
+        box.width = region.width - 4;
+        box.height = region.height - 4;
+    }
+    return updated_boxes;
+}
+
+}  // namespace freling