Merge two Pixbuf

[Tiberius Duluman <tiberius duluman gmail com>]

Hello,

I want to create a pixbuf that contains the overlapped images of Stock::Add
and a custom image.

I've tried to use Pixbuf::copy_area, but it does not work because it
doesn't care about transparency.

Also, I've tried to render one of the Pixbuf to Pixmap and Bitmap,
then to use Pixmap::draw_pixbuf with the second Pixbuf, but I have
no idea how to compose the correct mask.

Can somebody help me, please?