Make `geometry_instance` independent of `bottom_level_acceleration_structure`

[johannesugb]

It should be more obvious from the software design, that different top_level_acceleration_structures can reference the same bottom_level_acceleration_structures with individual instance data.

[johannesugb]

A preliminary solution has been developed. However, there should be some other (maybe additional?) way to manage geometry_instances and bottom_level_acceleration_structures which takes care of their lifetimes.

[johannesugb]

With VK_KHR_raytracing, this had to be restructured... but actually for the better, as it turns out. Needs to be revisited once again, however.