ライトで投影するクッキーを設定する。
Point lights require a cube map texture. If the cookie is a cube map, the light will become a Point light.
Note that cookies are only displayed for pixel lights.
EditorSettings.enableCookiesInLightmapper needs to be turned on for it to work for baked/mixed lights, otherwise it only works for real-time lights.
Additional resources: Light component.