Build PNG and JPEG loaders into gdk-pixbuf by default
Build PNG and JPEG loaders into gdk-pixbuf by default

GdkPixbuf without PNG and JPEG loaders is less than useful for loading
assets in GTK (and anywhere else, really).

We go to great lengths to ensure that both the PNG and JPEG loaders are
built by default, to the point of including subprojects for libpng and
libjpeg. Let's avoid any doubt, and ensure that those loaders are also
included in the default shared library.

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf