Support (out caller-allocates)

People have wanted support for marking (out) on functions of the
form:

/**
 * clutter_color_from_pixel:
 * @pixel: A pixel
 * @color: (out): Color to initialize with value of @pixel
 */
void
clutter_color_from_pixel (guint32 pixel, ClutterColor *color);

Where the caller is supposed to have allocated the argument; the
C function just initializes it.  This patch adds support for this
argument passing style to introspection.  In this case, we see the
(out), and notice that there's only a single indirection (*) on
the argument, and assume that this means (out caller-allocates).

https://bugzilla.gnome.org/show_bug.cgi?id=604749