Use child mesh in SplatMesh to auto-inject SparkRenderer instead of monkey-patching

[mrxz]

The current monkey-patching of THREE.Scene to auto-inject a SparkRenderer has many limitations and can easily break. The following setups cause issues that aren't immediately clear to the user:

• More than one THREE.Scene (<SparkRenderer> is optional but removing makes the splat disappear (R3F) #78 (comment))
• Using more than one canvas (Splats in diferent Canvas scopes issue #98, Multiple canvas/instances breaks #107)
• Adding a SparkRenderer or SplatMesh using .attach() instead of .add()
• Adding a SplatMesh as an indirect descendant of the scene, if one of the ancestors has already been added to the scene.
• Adding a SparkRenderer as an indirect descendant of the scene
• Using a custom scene.onBeforeRenderer

This PR introduces an alternative approach. Each SplatMesh adds an empty mesh to itself. During rendering it checks if the scene contains a SparkRenderer and if not injects one. Regardless of the outcome it will remove itself, ensuring the check is a one-time cost per SplatMesh.

This does mean that the following pattern is no longer allowed:

const spark = new SparkRenderer({ renderer });
// Never add it to the scene ourselves, yet expect it to take effect

Though, personally I think that's for the better. A few examples used this pattern, though it's unclear to me if this was intentional or not.