Edited Issue: Issue on opening large image (>2GB Tiff) [1403]

Hi,

We are using magick.net for getting some image informations (get colorspace, bit per sample, etc.) and in some case, we have to transform the same files (dropped in our main app) to tiff. For getting info, we use the code below. On opening large Tiff (>2GB Tiff), we get instantly an OverflowException, Arithmetic operation resulted in an overflow. at ImageMagick.MagickReader.Read(Stream stream)

Any idea we can overcome this. We currently use the V6 of magick.net. Q8 - X64

The whole project is X64 based, not anyCpu.

```
FileStream imageStream = new FileStream(TiffFile, FileMode.Open, FileAccess.Read);
MagickImage currentImage;
ImageMagick.ColorSpace orgColorspace;

try
{
currentImage = new MagickImage(imageStream);
orgColorspace = currentImage.ColorSpace;

if (currentImage.ReadWarning != null)
{
log.Info("RipTiff : Warning on opening file : " + TiffFile + " : " + currentImage.ReadWarning.Message);
}
}
```