Better custom vertex buffer layouts support

[trevex]

The goal of this PR is to make creating and using custom vertex buffer layouts easier and allow them to leverage a pre-existing VertexDefinition implementation. This will also allow vertex types derived from Vertex to interact with custom runtime created ones.

Here is some pseudo-code to illustrate the idea:

#[repr(C)]
#[derive(Clone, Copy, Debug, Default, Zeroable, Pod, Vertex)]
struct InstanceData {
    #[format(R32G32_SFLOAT)]
    position_offset: [f32; 2],
    #[format(R32_SFLOAT)]
    scale: f32,
}

// There is no runtime vertex buffer builder helper yet, but just image there is :)
const ATTRIBUTE_POSITION: VertexAttribute = VertexAttribute::new("position", Format::R32G32_SFLOAT);
const ATTRIBUTE_COLOR: VertexAttribute = VertexAttribute::new("color", Format::R32G32B32_SFLOAT);

let (buffer, info) = RuntimeVertexBufferBuilder::new()
        .add(
            ATTRIBUTE_POSITION,
            vec![
                Vec2::new(-0.5, -0.25),
                Vec2::new(0.0, 0.5),
                Vec2::new(0.25, -0.1),
            ],
        )
       .build();

// Both can be used together and leverage the matching logic previously implemented by `BuffersDefinition`
let pipeline = GraphicsPipeline::start()
        .vertex_input_state([info.per_vertex(), InstanceData::per_instance()])

1. Update documentation to reflect any user-facing changes - in this repository.

2. Make sure that the changes are covered by unit-tests.

3. Run cargo fmt on the changes.

4. Please put changelog entries in the description of this Pull Request
   if knowledge of this change could be valuable to users. No need to put the
   entries to the changelog directly, they will be transferred to the changelog
   file by maintainers right after the Pull Request merge.

   Please remove any items from the template below that are not applicable.

5. Describe in common words what is the purpose of this change, related
   Github Issues, and highlight important implementation aspects.

Changelog:

### Breaking changes
Deprecates `BuffersDefinition`:
- Use `VertexBufferDescription` directly.

### Additions
- Introduction of `VertexBufferDescription` makes it easy to create custom description and leverage the matching logic previously provided by `BuffersDefinition`.

While this change does not directly implement runtime interleaved vertex buffers. There is nothing stopping users from implementing them as demonstrated in Discord by @marc0246 and on the runtime-vertex-* branches of my fork.

Fixes #736

[trevex]

Updated the description and reimplemented BuffersDefinition, although it would be deprecated by these changes.

If you look at the code please careful review the names I have chosen for structs. I am not sure the *Info is a great idea considering they have no relation to Vulkan and might confuse users, but I have yet to come up with better ones. Opinions appreciated.

[trevex]

Thanks @Rua, I will go through your feedback.

As the changes of this PR make the implementation of runtime vertex formats possible I wanted to give it a try. The last commit implements a way to create interleaved runtime vertex formats and buffers. I added a simple test and example and seems to work fine.

Non-interleaved vertex buffers would already be possible either way, but this fixes the open issues regarding interleaved runtime buffers.

I expect the code to need careful review regarding allocations and the iterator logic.

Should I include those as part of this PR or move those changes elsewhere?

[Rua]

The runtime stuff is probably better left to a separate PR.

If this is meant to deprecate BuffersDefinition, can you add a deprecation notice to it? And can you add documentation to the methods of Vertex and VertexBufferInfo?

[trevex]

Before continuing to update the examples:
I would like to get some feedback on the struct-name VertexBufferInfo. Would VertexFormat or VertexBufferFormat be a better name?

[Rua]

It seems to describe the contents of a vertex buffer, so how about VertexBufferDescription?