It's composed by
- border-image-source: a cairo_pattern_t holding an image from file or a
  gradient
- border-image-slice: a GtkBorder containing the slice offsets to apply
  on the image
- border-image-repeat: a GtkRepeatType for the image

We deviate from pure CSS3 in the following ways:

* border-image-width is assumed to be always 1, i.e. always equal to
  what's specified by border-width. I don't think it's a particularly
  useful property to have, but we could add it later if needed.
* border-image-outset is absent, as we can't render outside of the
  allocation yet.

https://bugzilla.gnome.org/show_bug.cgi?id=651194