spotlight map alpha

[mbredif]

This PR is a follow-up to #21944 on textured spot lights.
It enables the use of spotlight textures with and without being affected by the spotlight cone and penumbra. This is controlled (=mixed) by the alpha value of the texture.

• if the texture is transparent, the spotlight cone is deactivated
• if the texture is opaque, the texture is modulated by the spotlight cone lighting

This example uses a texture that is half transparent (bottom of the screen) and half opaque (top).

[mrdoob]

Sorry, I do not think this is the way to go... 😕

All these caveats in the docs are a sign that the API is not intuitive.