Creates a new pixbuf by asynchronously loading an image from an input stream.
For more details see Pixbuf.newFromStreamAtScale, which is the synchronous version of this function.
When the operation is finished, callback will be called in the main thread. You can then call Pixbuf.newFromStreamFinish to get the result of the operation.
a gio.InputStream from which to load the pixbuf
the width the image should have or -1 to not constrain the width
the height the image should have or -1 to not constrain the height
TRUE to preserve the image's aspect ratio
optional gio.Cancellable object, NULL to ignore
a GAsyncReadyCallback to call when the pixbuf is loaded
the data to pass to the callback function