Use the right conversion from cairo to GdkPixbuf

Use the right conversion from cairo to GdkPixbuf

GdkPixbuf is stored in memory as individual bytes, in strict red-green-blue-alpha
order, while cairo image surfaces are stored as 32bits units whose
order depends on the endianess.
It is probably possible to do something better, taking advantage
of cogl and the GL using the actual component order, but for now
it is easier to use the GDK utility to convert the cairo surface.

https://bugzilla.gnome.org/show_bug.cgi?id=693931