We're mainly using this for icons that will never change contents or
size, and we're using a GtkScaler per such icon when on a hidpi setup,
so just avoid the two signal connections per icon.
Instead of loading them into surfaces (which we want to get rid of), we
load into textures.
In fact, we introduce a new paintable subclass called a GtkScaler that
takes care of tracking scaling.
This also ideally gets rid of an extra conversion once renderers learn
to render textures directly.
