We render the source into a cairo_surface_t so that we can render it
with cairo directly, rather than having to convert it from a pixbuf
every time. We also specify the target window when creating the cairo
surface so that rendering can be faster.
Using cairo surfaces also allows us to seamlessly support window scales.
We also add a GTK_IMAGE_SURFACE source type.
There are some registred stock ids like gtk-discards that have no icons,
and you could also pass a non-registred stock id. Both of these means
gtk_style_context_lookup_icon_set returns NULL, which causes
a critical in gtk_icon_set_render_icon_pixbuf.
We avoid this by just making these render as EMPTY.
Treat the empty string as if it was a NULL icon name instead of
rendering GTK_STOCK_MISSING_IMAGE.
This used to be the previous behavior of GtkCellRendererPixbuf and
applications might still rely on it.
GtkIconHelper is a helper object to easily obtain a pixbuf from
different icon sources (e.g. a GIcon, an icon name, a stock id, ...).
Code is ported from GtkImage, which will be adapted in the next commit.
