Add new ImageLoaderSettings member for loading as stacked 2d array

[KirmesBude]

Objective

Loading images for use as texture 2d array is tedious and should be supported in the context of loading the asset.
Partially fixes #17145

Solution

Add a new layers member to ImageLoaderSettings which determines if and how we call reinterpret_stacked_2d_as_array during asset loading.

Testing

• Did you test these changes? If so, how?
  • I ran ci locally and simplified and ran the current texture 2d array example
• Are there any parts that need more testing?
  • I do not think so
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
  • Run array_texture example
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?
  • linux, amd cpu, amd gpu

---

Showcase

I believe the changes in examples/shader/array_texture.rs show quite well how this is nicer to work with.

[alice-i-cecile]

Very nice!

[KirmesBude]

I also changed the tilemap example. I see one use of reinterpret_stacked_2d_as_array in the skybox example. Maybe it is better to use something like TextureViewDimension as suggested in the issue to also cover the cubemap case (and possibly more).

[KirmesBude]

I just realized this actually panics. So I want to change this so it returns a Result, but I am not sure if I should keep the panicing version? Is there any benefit to it?

[alice-i-cecile]

Please swap it to return a Result. Panicking APIs are only worth it when the operation is extremely common and the panics are extremely rare. Neither is the case here :)