gtk_css_style_get_value does not return a reference. So don't unref the return value without taking one.
If we fail to load the image for a -gtk-recolor() expression, fall back to using the image-missing icon instead of crashing, and include more details in the warning message.
This can come in handy to load images that are not in an icon theme, but still benefit from symbolic recoloring. https://bugzilla.gnome.org/show_bug.cgi?id=760536