Make vertex buffer slots optional

[teoxoy]

Connections
Fixes #7354.
Fixes #8832.
Fixes #7912.

Description
Makes vertex buffer slots optional.

Testing
Enabled more CTS tests.

Squash or Rebase?
Rebase.

[andyleiserson]

Nice! Just a few nits.

[andyleiserson]

I took a stab at deduplicating this for passes and bundles. It needs a bunch of internal types from command so possibly it's worth creating a new module command::validation rather than putting this in crate::validation.

pub(crate) fn validate_vertex_buffers(
    pipeline: &Arc<crate::pipeline::RenderPipeline>,
    slot_mask: BitVec,
    binder: &crate::command::bind::Binder,
) -> Result<(), DrawError> {
    // Check all needed vertex buffers have been bound
    for index in pipeline.vertex_steps
        .iter()
        .enumerate()
        .filter_map(|(index, step)| step.map(|_| index))
    {
        if slot_mask.get(index) != Some(true) {
            return Err(DrawError::MissingVertexBuffer {
                pipeline: pipeline.error_ident(),
                index,
            });
        }
    }

    let bind_group_space_used = binder.last_assigned_index().map_or(0, |i| i + 1);
    let vertex_buffer_space_used = slot_mask
        .iter()
        .enumerate()
        .filter_map(|(i, size)| size.then_some(i))
        .next_back()
        .map_or(0, |i| i + 1);

    let bind_groups_plus_vertex_buffers =
        u32::try_from(bind_group_space_used + vertex_buffer_space_used).unwrap();
    if bind_groups_plus_vertex_buffers
        > pipeline.device.limits.max_bind_groups_plus_vertex_buffers
    {
        return Err(DrawError::TooManyBindGroupsPlusVertexBuffers {
            given: bind_groups_plus_vertex_buffers,
            limit: pipeline.device.limits.max_bind_groups_plus_vertex_buffers,
        });
    }

    Ok(())
}

// Pass
validation::validate_vertex_buffers(
    &pipeline,
    BitVec::from_iter(self.vertex.buffer_sizes.iter().map(|size| size.is_some())),
    &self.pass.binder,
)?;

// Bundle
validation::validate_vertex_buffers(
    &pipeline.pipeline,
    BitVec::from_iter(self.vertex.iter().map(|vbs| vbs.is_some())),
    &self.binder,
)?;

[andyleiserson]

It might be worth using expect here.

[teoxoy]

There are quite a few unwraps in this function, mainly because we are reusing the same RenderCommand type which has variants with optional fields. I think we should probably use a different type for the render bundle commands after they have been validated. It would be in line with our recent talks about splitting internal from external types.