gegl-gtk is not an actively maintained or widely distributed library, but it did give us a head start with rendering a GeglNode inside a GtkWidget. This is why we carry our own copy of the code.

Over the years, our copy of gegl-gtk had diverged quite a bit:
 - We don't use a processor inside the widget.
 - Our AUTOSCALE_CONTENT logic always centres the image.
 - Sharper renders on HiDpi displays.
 - We don't care about GTK+ 2.x or the absence of Cairo.
    
The widget / helper split is also largely useless for us. While it will be good to have tests for the automatic centring and scaling code, we don't need a separate helper class for it.
    
While some of our changes can be upstreamed, we are ultimately going to use the new image viewer widget in GTK+. We are also thinking of using transitions when the output of the GeglNode changes. Having only the bits that we need in a single class, and getting rid of the extra fluff will help us evolve this code in future.

Otherwise we are stuck with a class that is, in theory, meant to be used by multiple different applications and hence should have a nice API, while in reality we are the only users and there is barely any "upstream" maintenance.

Since PhotosImageView largely constitutes code written by me, we can license it as GPLv2+, unlike gegl-gtk, which was LGPLv3+.