background: Composite the emblems ourselves

We were assuming that setting stock-size would affect the emblems in
GEmblemedIcons, but not the icons themselves. This is a bit weird.
GtkCellRendererPixbuf:gicon is meant to work with
GtkCellRendererPixbuf:stock-size, and this was only working so far
because GTK_ICON_LOOKUP_FORCE_SIZE was not being used when loading
the icon.

Let's composite the emblems ourselves so that we don't have to depend
on this quirky interpretation of stock-size.

Unfortunately, we can not directly use the pixbufs because they are
unaware of the scale factor and GTK+ will scale them on HiDpi
displays. Since our pixbufs already have enough pixels to work well
with such devices, scaling them further will lead to giant, fuzzy
thumbnails. Hence, we use GtkCellRendererPixbuf:surface with the
scale factor codified in it.

https://bugzilla.gnome.org/show_bug.cgi?id=732375