移動先のレンダーテクスチャ
Usually cameras render directly to screen, but for some effects it is useful to make
a camera render into a texture. This is done by creating a RenderTexture object
and setting it as targetTexture on the camera. The camera will then render into
that texture.
targetTexture が null を返す場合、カメラは画面をレンダリング先にします。
When rendering into a texture, the camera always renders into the whole texture; effectively
rect and pixelRect are ignored.
It is also possible to make camera render into separate RenderBuffers, or into
multiple textures at once, using SetTargetBuffers function.
Additional resources: RenderTexture, SetTargetBuffers.