Metal descriptor set write spill is broken

[zakarumych]

As this note stated descriptors must be bound to following bindings when there are more provided than array count of the binding point. Yet this is true only for Vulkan backend currently. This could be a problem for portability.

[msiglreith]

dx12 and metal are doing this as well iirc. I think @kvark implemented it some time ago

[kvark]

I do recall actually covering this case. @omni-viral do you see the logic missing in D3D12/metal at the moment?

[zakarumych]

Yes. It doesn't work on metal. I'll recheck

[zakarumych]

I confim. On metal.

This code works.

factory.write_descriptor_sets(
    once(DescriptorSetWrite {
        set: &set,
        binding: 0,
        array_offset: 0,
        descriptors: Some(Descriptor::Buffer(buffer.borrow(), Some(tilemap_uniform_range.start) .. Some(tilemap_uniform_range.end))),
    }).chain(once(DescriptorSetWrite {
        set: &set,
        binding: 1,
        array_offset: 0,
        descriptors: Some(Descriptor::Buffer(buffer.borrow(), Some(transform_uniform_range.start) .. Some(transform_uniform_range.end))),
    })).chain(once(DescriptorSetWrite {
        set: &set,
        binding: 2,
        array_offset: 0,
        descriptors: Some(Descriptor::Sampler(&self.sampler)),
    })).chain(once(DescriptorSetWrite {
        set: &set,
        binding: 3,
        array_offset: 0,
        descriptors: Some(Descriptor::Image(self.tileset.view(), Layout::ShaderReadOnlyOptimal)),
    }))
);

This segfaults

factory.write_descriptor_sets(
    once(DescriptorSetWrite {
        set: &set,
        binding: 0,
        array_offset: 0,
        descriptors: &[
            Descriptor::Buffer(buffer.borrow(), Some(tilemap_uniform_range.start) .. Some(tilemap_uniform_range.end)),
            Descriptor::Buffer(buffer.borrow(), Some(transform_uniform_range.start) .. Some(transform_uniform_range.end)),
            Descriptor::Sampler(&self.sampler),
            Descriptor::Image(self.tileset.view(), Layout::ShaderReadOnlyOptimal),
        ],
    })
);