Many tiff files (such as those created by Photoshop CS) contain
multiple alpha channels that are not associated to the pixel data.
ImageGlue only allows one alpha channel to be associated with the
image data, this parameter allows the user to specify which alpha
channel to use.
The index begins at one and a value of zero is used to indicate
no alpha channel.
The default behavior is: If a tiff file contains alpha channels,
and the tiff 'extrasamples' tag does not indicate that these alpha
channels are associated with the pixel data, ImageGlue will ignore