Consolidate Render(Ui)Materials(2d) into RenderAssets

[superdump]

Objective

• Replace RenderMaterials / RenderMaterials2d / RenderUiMaterials with RenderAssets to enable implementing changes to one thing, RenderAssets, that applies to all use cases rather than duplicating changes everywhere for multiple things that should be one thing.
• Adopts Improve render asset #8149

Solution

• Make RenderAsset generic over the destination type rather than the source type as in Improve render asset #8149
• Use RenderAssets<PreparedMaterial<M>> etc for render materials

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Changelog

• Changed:
  • The RenderAsset trait is now implemented on the destination type. Its SourceAsset associated type refers to the type of the source asset.
  • RenderMaterials, RenderMaterials2d, and RenderUiMaterials have been replaced by RenderAssets<PreparedMaterial<M>> and similar.

Migration Guide

• RenderAsset is now implemented for the destination type rather that the source asset type. The source asset type is now the RenderAsset trait's SourceAsset associated type.

[superdump]

I would love to find a way to not require people to implement AssetUsages for custom materials, and instead just use a default. But I haven't had any good ideas yet. I can't implement for AssetUsages for T at least. And not every Asset should be a RenderAsset.

[infmagic2047]

I would love to find a way to not require people to implement AssetUsages for custom materials, and instead just use a default. But I haven't had any good ideas yet. I can't implement for AssetUsages for T at least. And not every Asset should be a RenderAsset.

My ideas: Keep the asset_usage() method on RenderAsset instead of using a separate trait (it should take &Self::SourceAsset param now). Add methods to Material traits that allow overriding the usage, and use them in the RenderAsset implementation of PreparedMaterial. The required migration would be minimal in this way.

[superdump]

@infmagic2047 great idea! So simple! :) I love it.

[pcwalton]

This looks fine. I'll trust you that it was necessary to move RenderAsset over to be implemented on the render world type.

[IceSentry]

@pcwalton here's the justification from the original PR:

Remove one to one correlation between "main world" asset and GPU asset, by swapping base type that implements RenderAsset to be the GPU asset type. That would make RenderAssetPlugin more flexible, being able to prepare different data based on same source asset (example: effect plugin could preprocess mesh buffer data for specialized rendering, without affecting main render flow).
That also makes RenderAssets more explicit.

[superdump]

Please don’t merge this yet. I want to think about a couple of things with it so I am convinced implementing RenderAsset on the destination type is better.

[superdump]

I turned out that robtfm had a branch that did basically the same thing last year after I commented that implementing RenderAsset on types that implement Materials was problematic in various ways:

• Problems I discovered: https://discord.com/channels/691052431525675048/743663924229963868/1150911939472523315
• robtfm's branch: https://discord.com/channels/691052431525675048/743663924229963868/1150933597977591809