Returns a WritableRaster representing the alpha channel for BufferedImage objects with ColorModel objects that support a separate spatial alpha channel, such as ComponentColorModel and DirectColorModel. Returns null if there is no alpha channel associated with the ColorModel in this image. This method assumes that for all ColorModel objects other than IndexColorModel, if the ColorModel supports alpha, there is a separate alpha channel which is stored as the last band of image data. If the image uses an IndexColorModel that has alpha in the lookup table, this method returns null since there is no spatially discrete alpha channel. This method creates a new WritableRaster, but shares the data array.

Returns:  a WritableRaster or null if this BufferedImage has no alpha channel associated with its ColorModel.