Generate D2D1 input descriptions as file local ReadOnlyMemory<T>

[Sergio0694]

Description

This PR updates the internal ID2D1Shader interface and generator to emit input descriptions as a ReadOnlyMemory<T>:

namespace ComputeSharp.D2D1.__Internals;

public interface ID2D1Shader
{
    ReadOnlyMemory<D2D1InputDescription> InputDescriptions { get; }
}

This also removes the previously used ID2D1InputDescriptionsLoader interface.