- Vertex buffer layouts are set on `Mesh` for shader specialization.
  `Mesh` knows how to specialize its own vertex buffer because it owns
  the vertex data.
- The `RenderAsset` implementation for `Mesh` caches the vertex buffer
  layout and adds a cache key to the `GpuMesh` render asset. The
  `MeshPipeline` can lookup the vertex buffer layout by composing
  `VertexLayoutKey` within `MeshPipelineKey`.
- `SpritePipeline` and `UiPipeline` populates the vertex layout cache
  for themselves..
- `SpecializedPipeline::specialize` now takes a reference to the
  `RenderPipelineCache`, which is the best way I could find to get a
  reference to the cache for vertex layout lookups.
- Discussion: https://discord.com/channels/691052431525675048/743663924229963868/908484759833960489

Co-authored-by: Nicola Papale <nicopap@users.noreply.github.com>