texture-cache: use scale factor to load sliced image

We need to use a GdkPixbufLoader instead of the straightforward
gdk_pixbuf_new_from_file(), since we want to load the image already
scaled if possible - e.g. if it's an SVG file.

https://bugzilla.gnome.org/show_bug.cgi?id=726907
texture-cache: always call gdk_pixbuf_loader_set_size

Just use the regular image size if we don't want to scale it. This is a
refactor in preparation for the next commit.

https://bugzilla.gnome.org/show_bug.cgi?id=726907
border-image: add support for scale factor

In a later commit we'll add support for rendering borders and
backgrounds with scale factor.

https://bugzilla.gnome.org/show_bug.cgi?id=726907
texture-cache: use scale factor to load background and borders

https://bugzilla.gnome.org/show_bug.cgi?id=726907
texture-cache: use scale factor for load_uri_async()

https://bugzilla.gnome.org/show_bug.cgi?id=726907