RectAreaLightHelper: Ignore inherited scale factors

[WestLangley]

RectAreaLights are sized by light.width and light.height, and ignore any inherited scaling from the scene graph.

Typically, lights are not scaled, but a light could be a child of a scaled object.

This PR ensures that RectAreaLightHelper ignores inherited scaling, too, and renders the area light in its specified size.

[WestLangley]

Related issue:

DirectionalLightHelper's so-called "handle" is affected by world scale, but there is a scale parameter in the helper constructor that can offset that.

[WestLangley]

The glTF spec says

The light's transform is affected by the node's world scale, but all properties of the light (such as range and intensity) are unaffected.

Similarly, in three.js, the size of the rect area light is not affected by the world scale.

[mrdoob]

Thanks!