[Merged by Bors] - Modular Rendering

[cart]

This changes how render logic is composed to make it much more modular. Previously, all extraction logic was centralized for a given "type" of rendered thing. For example, we extracted meshes into a vector of ExtractedMesh, which contained the mesh and material asset handles, the transform, etc. We looked up bindings for "drawn things" using their index in the Vec<ExtractedMesh>. This worked fine for built in rendering, but made it hard to reuse logic for "custom" rendering. It also prevented us from reusing things like "extracted transforms" across contexts.

To make rendering more modular, I made a number of changes:

• Entities now drive rendering:
  • We extract "render components" from "app components" and store them on entities. No more centralized uber lists! We now have true "ECS-driven rendering"
  • To make this perform well, I implemented [Merged by Bors] - Spawn specific entities: spawn or insert operations, refactor spawn internals, world clearing #2673 in upstream Bevy for fast batch insertions into specific entities. This was merged into the pipelined-rendering branch here: [Merged by Bors] - Add upstream bevy_ecs and prepare for custom-shaders merge  #2815
• Reworked the Draw abstraction:
  • Generic PhaseItems: each draw phase can define its own type of "rendered thing", which can define its own "sort key"
  • Ported the 2d, 3d, and shadow phases to the new PhaseItem impl (currently Transparent2d, Transparent3d, and Shadow PhaseItems)
  • Draw trait and and DrawFunctions are now generic on PhaseItem
  • Modular / Ergonomic DrawFunctions via RenderCommands
    • RenderCommand is a trait that runs an ECS query and produces one or more RenderPass calls. Types implementing this trait can be composed to create a final DrawFunction. For example the DrawPbr DrawFunction is created from the following DrawCommand tuple. Const generics are used to set specific bind group locations:

       pub type DrawPbr = (
          SetPbrPipeline,
          SetMeshViewBindGroup<0>,
          SetStandardMaterialBindGroup<1>,
          SetTransformBindGroup<2>,
          DrawMesh,
      );

    • The new custom_shader_pipelined example illustrates how the commands above can be reused to create a custom draw function:

      type DrawCustom = (
          SetCustomMaterialPipeline,
          SetMeshViewBindGroup<0>,
          SetTransformBindGroup<2>,
          DrawMesh,
      );

• ExtractComponentPlugin and UniformComponentPlugin:
  • Simple, standardized ways to easily extract individual components and write them to GPU buffers
• Ported PBR and Sprite rendering to the new primitives above.
• Removed staging buffer from UniformVec in favor of direct Queue usage
  • Makes UniformVec much easier to use and more ergonomic. Completely removes the need for custom render graph nodes in these contexts (see the PbrNode and view Node removals and the much simpler call patterns in the relevant Prepare systems).
• Added a many_cubes_pipelined example to benchmark baseline 3d rendering performance and ensure there were no major regressions during this port. Avoiding regressions was challenging given that the old approach of extracting into centralized vectors is basically the "optimal" approach. However thanks to a various ECS optimizations and render logic rephrasing, we pretty much break even on this benchmark!
• Lifetimeless SystemParams: this will be a bit divisive, but as we continue to embrace "trait driven systems" (ex: ExtractComponentPlugin, UniformComponentPlugin, DrawCommand), the ergonomics of (Query<'static, 'static, (&'static A, &'static B, &'static)>, Res<'static, C>) were getting very hard to bear. As a compromise, I added "static type aliases" for the relevant SystemParams. The previous example can now be expressed like this: (SQuery<(Read<A>, Read<B>)>, SRes<C>). If anyone has better ideas / conflicting opinions, please let me know!
• RunSystem trait: a way to define Systems via a trait with a SystemParam associated type. This is used to implement the various plugins mentioned above. I also added SystemParamItem and QueryItem type aliases to make "trait stye" ecs interactions nicer on the eyes (and fingers).
• RenderAsset retrying: ensures that render assets are only created when they are "ready" and allows us to create bind groups directly inside render assets (which significantly simplified the StandardMaterial code). I think ultimately we should swap this out on "asset dependency" events to wait for dependencies to load, but this will require significant asset system changes.
• Updated some built in shaders to account for missing MeshUniform fields

[superdump]

I love all of this and as @cart knows I built the visibility culling stuff I've been working on on top of it. Merge plz. 😄 ❤️

[Weibye]

Examples needs to be added to examples/README.md

:)

[cart]

bors r+

[bors]

Pull request successfully merged into pipelined-rendering.

Build succeeded:

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