Use g_memdup2()

GLib 2.67.3 introduced g_memdup2() to fix a type signature issue with g_memdup().

GTK uses g_memdup() mostly with size_t and guint arguments, except in a couple of cases where we mix int and size_t.

There's no real need to bump up the required version of GLib, so there's a shim that should cover us until we can depend on GLib 2.68 for other reasons.