Add new numeric properties to ID2D1Shader

[Sergio0694]

Description

This PR adds two new numeric properties to ID2D1Shader and exposes them in D2D1PixelShader:

API breakdown

namespace ComputeSharp.D2D1.Interop
{
    public static class D2D1PixelShader
    {
        public static int GetResourceTextureCount<T>()
            where T : unmanaged, ID2D1PixelShader;
    }
}

namespace ComputeSharp.D2D1.__Internals
{
    public interface ID2D1Shader
    {
        int ConstantBufferSize { get; }
        int ResourceTextureCount { get; }
    }
}