Propose: Add the async Write in IPDFObject

[lindexi]

VectSharp/VectSharp.PDF/PDFObjects/IPDFObject.cs

Lines 32 to 40 in b03e0c6

	public interface IPDFObject
	{
	/// <summary>
	/// Writes the <see cref="IPDFObject"/> to a stream.
	/// </summary>
	/// <param name="stream">The stream on which the <see cref="IPDFObject"/> should be written.</param>
	/// <param name="writer">A <see cref="StreamWriter"/> used to write text to the stream. Always call <see cref="StreamWriter.Flush"/> before returning from this method!</param>
	void Write(Stream stream, StreamWriter writer);
	}

How about add the WriteAsync to IPDFObject?

    public interface IPDFObject
    {
        /// <summary>
        /// Writes the <see cref="IPDFObject"/> to a stream.
        /// </summary>
        /// <param name="stream">The stream on which the <see cref="IPDFObject"/> should be written.</param>
        /// <param name="writer">A <see cref="StreamWriter"/> used to write text to the stream. Always call <see cref="StreamWriter.Flush"/> before returning from this method!</param>
        void Write(Stream stream, StreamWriter writer);
+       Task WriteAsync(Stream stream, StreamWriter writer);
    }