image: support scale factor when loading from GResource and file

image: support scale factor when loading from GResource and file

Currently, when loading an image from a GResource or file we don't take
the scale factor of the display into consideration, and let
GtkIconHelper scale it accordingly.
While this in general works for non-scalable images, we can take
advantage of the native loader's scaling for e.g. SVG images, and load
them at the right scale factor automatically.
This is achieved by switching to a pixbuf loader instead of using the
native function.

https://bugzilla.gnome.org/show_bug.cgi?id=733416