Only enable shadowMap if an entity uses the shadow component and the shadow system is enabled

[mrxz]

Description:
The shadow system has logic for only enabling once an entity uses shadows. However, this logic is broken since #4040, resulting in renderer.shadowMap.enabled to always default to true. This means that by default every A-Frame scene renders an empty shadowMap and compiles the materials with USE_SHADOWMAP defined.

On top of that, while the system would update renderer.shadowMap.enabled when changing the system's enabled property, this alone isn't enough to actually toggle the shadows in Three.js. It's needed for materials to be updated whenever shadowMap.enabled changes (How to update things).

This PR addresses both issues, restoring the original intended behaviour and ensuring enabling and disabling the shadow system on the fly works as well.

Changes proposed:

• Use this.data.enabled for if the shadow system is enabled and this.shadowMapEnabled for if a shadowMap is needed (e.g. an entity uses the shadow component). As a result renderer.shadowMap.enabled = this.data.enabled && this.shadowMapEnabled
• Mark all materials as needsUpdate whenever the actual value of renderer.shadowMap.enabled changes
• Update unit tests to reflect the above changes

[dmarcos]

Maybe this can be factored out to an independent function (example) at the bottom of the file. updateAllMaterials(sceneEl)

[mrxz]

Done

[dmarcos]

Thanks so much. You rock 🎸