QImageReader should support incremental loading of large images

Since at least the GTK+ 2.x era, GdkPixbufLoader's area-prepared and area-updated signals, have enabled a form of asynchronous image loading which allows users of tools like GQView (now Geeqie) to see and react to the parts of an image that have loaded before the load process finishes.

(This is very useful for using a scroll wheel to scrub through a collection of images because, not only does it allow the user to make judgments based on a partially loaded image, it also allows the UI to avoid feeling janky if they are rapidly scrolling through a set of images as a means of approximating PgUp/PgDown motion using the mouse and they encounter an unusually large image. Both things Gwenview, KDE's closest equivalent to GQView, has historically suffered from.)

The closest equivalent that I've been able to find in Qt is the ability for QImageReader backends to clip the image as part of the loading process, which has much less format support than GdkPixbufLoader appears to support and is inherently going to be heavier than periodically dispatching a signal containing a reference to the QImage that's in the process of having its backing store populated.

(Though I did find evidence of interest in going back at least to 2009.)