Remove duplications

==== old code
public static void WritePts(this Stream stream, ulong pts)
{
    //TODO: check max
    var buffer = BitConverter.GetBytes(pts);
    if (BitConverter.IsLittleEndian)
    {
        stream.WriteByte(buffer[4]);
        stream.WriteByte(buffer[3]);
        stream.WriteByte(buffer[2]);
        stream.WriteByte(buffer[1]);
        stream.WriteByte(buffer[0]);
    }
    else
    {
        stream.WriteByte(buffer[buffer.Length - 1]);
        stream.WriteByte(buffer[buffer.Length - 2]);
        stream.WriteByte(buffer[buffer.Length - 3]);
        stream.WriteByte(buffer[buffer.Length - 4]);
        stream.WriteByte(buffer[buffer.Length - 5]);
    }
}

==== new code
public static void WritePts(this Stream stream, ulong pts)
{
    var buffer = BitConverter.GetBytes(pts);

    int startIndex = BitConverter.IsLittleEndian ? 0 : buffer.Length - 5;

    for (int i = 4; i >= 0; --i)
    {
        stream.WriteByte(buffer[startIndex + i]);
    }
}