Rework descriptor setup

[SaschaWillems]

In the texture mapping chapter, the tutorial uses one one combined image sampler descriptor per frame in flight (same as uniform buffers). But unlike the uniform buffer, the combined image sampler will never be updated once the image has been loaded, so duplicating the descriptor per frame in flight feels odd and is kinda bad practice. We should rework that chapter and properly decouple the uniform buffer descriptors from the image descriptors using separate descriptor sets. This is then also a good spot for telling people about how to separate descriptors "per update rate".