Fix ref counting and originToWorld in SparkViewpoint.prepare

[mrxz]

See #205

The prepare method of SparkViewpoint increased the refCount of the active accumulator in case it wasn't used in the viewpoint's display state. However the display reference is already covered, resulting in double counting this reference. At the same time there's no guarantee the viewpoint holds any reference when sortUpdate is called. To resolve this a temporary reference is held.

Additionally there was a bug where the prepare method would default to not providing an originToWorld matrix. This causes the SparkRenderer to use the current toWorld of the active accumulator. This works as long as the SparkRenderer hasn't been moved, but renders incorrectly in case it did. See following scenario:

// Move camera with spark renderer
camera.position.x += 10.0;
await viewpoint.prepare({ scene, camera }); // Prepares for an outdated SparkRenderer position
renderer.render(scene, camera); // Triggers a new update, renders incorrectly

One thing that remains tricky with regards to prepare is that it directly calls sortUpdate, ignoring any ongoing sort operation or pending sort operation. This makes it possible for sortUpdate to throw an error, breaking the ref counting.

[dmarcos]

Thank you!