Change GpuImage::size from UVec2 to Extent3d

[Noxmore]

Objective

When preparing GpuImages, we currently discard the depth_or_array_layers of the Image's size by converting it into a UVec2.

Solution

Change GpuImage::size to Extent3d, and just pass that through when creating GpuImages.
Also copy the aspect_ratio, and size (now size_2d for disambiguation from the field) functions from Image to GpuImage for ease of use with 2D textures.
I originally copied all size-related functions (like width, and height), but i think they are unnecessary considering how visible the size field on GpuImage is compared to Image.

Testing

Tested via cargo r -p ci for everything except docs, when generating docs it keeps spitting out a ton of

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> crates/bevy_dylib/src/lib.rs:1:21
  |
1 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
  | 

Not sure why this is happening, but it also happens without my changes, so it's almost certainly some strange issue specific to my machine.

Migration Guide

• GpuImage::size is now an Extent3d. To easily get 2D size, use size_2d().

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[alice-i-cecile]

Very straightforward. I agree with discarding the width etc helpers :)

[JMS55]

To double check my understanding, Image's already used Extent3d for all their sizing stuff, and it was only GpuImage that was locked to 2d?

[Noxmore]

To double check my understanding, Image's already used Extent3d for all their sizing stuff, and it was only GpuImage that was locked to 2d?

Yep.
Though, while i was looking around, i found ManualTextureView to also use a UVec2 for size, but i don't know much about that, and i figured it was probably out of scope of this PR anyway.