Describe the pull request

This PR slightly reworks descriptor set layouts and their allocation. First of all, it is now more easy to allocate descriptor sets without providing a resource to bind to it directly. The resource in this case needs to be bound before using the descriptor set later:

auto& geometryPipeline = m_device->state().pipeline("Geometry");
auto& cameraBindingLayout = geometryPipeline.layout()->descriptorSet(DescriptorSets::Constant);
auto cameraBindings = cameraBindingLayout.allocate();
// ...
auto cameraBuffer = m_device->factory().createBuffer("Camera", cameraBindingLayout, 0, BufferUsage::Resource);
cameraBindings->update(*cameraBuffer);

This is enabled by a std::monostate overload for the descriptor binding resource container, which together with the binding now being optional allows for default-constructable DescriptorBindings. This second change to the binding property allows to use pre-ordered indices for register bindings:

auto transformBindings = transformBindingLayout.allocate({
    { .resource = *transformBuffer, .firstElement = 0, .elements = 0 }, // Binds to space 0, descriptor 0
    { .resource = *transformBuffer, .firstElement = 0, .elements = 1 }, // Binds to space 0, descriptor 1
    { .resource = *transformBuffer, .firstElement = 0, .elements = 1 }  // Binds to space 0, descriptor 2
});

Note that a single DescriptorBinding refers to one descriptor within a descriptor set. For allocateMultiple this means that counting descriptor indices is reset for each element of the outer collection.

Additionally, t is now valid for the Vulkan backend to define empty descriptor sets. Earlier this would have resulted in validation errors and failures when attempting to bind the descriptor set. When creating a pipeline layout, empty descriptor sets are created for spaces that have no layout associated with them (up until the layout with the highest space index). The following shader can now be used for both backends:

ConstantBuffer<Camera> camera : register(b1, s1);
ConstantBuffer<Transform> transform : register(b0, s2);

The space indices are now validate for both backends. Within one pipeline each descriptor set layout must use distinct spaces. If two layouts share the same space, an error will be raised when attempting to create the pipeline layout.

Related issues

• Closes Allow empty and undefined descriptor sets. #106.
• Closes Allow descriptor set allocation without pre-existing buffers. #107.