public (int VertexOffset, int VertexCount) AddVertices(ReadOnlySpan <Byte> vertexData, VertexDeclaration vertexFormat)
        {
            var count = vertexData.Length / vertexFormat.VertexStride;

            if (count == 0)
            {
                return(_VertexCount, 0);
            }

            if (_VertexCount == 0)
            {
                _VertexElements = vertexFormat.GetVertexElements();
                _VertexStride   = vertexFormat.VertexStride;
            }
            else
            {
                var thisVD = new VertexDeclaration(_VertexStride, _VertexElements);
                if (!thisVD.Equals(vertexFormat))
                {
                    throw new ArgumentException(nameof(vertexFormat));
                }
            }

            vertexData = vertexData.Slice(0, count * _VertexStride);

            int offset = _VertexCount;

            Array.Resize(ref _VertexData, (offset + count) * _VertexStride);
            vertexData.CopyTo(_VertexData.AsSpan().Slice(offset * _VertexStride));

            _VertexCount += count;

            return(offset, count);
        }