r5: Make plants clickable through transparent region of bounding wireframe

[譲葉瑠珈]

Current selection box is shown as a wireframe, but is actually a CubeGeometry. So we can't click through its transparent region, which is unintuitive.

Replace CubeGeometry Mesh with something else.

[譲葉瑠珈]

This not so problematic since AABB is now fitted to the plant size.