GtkImage expose is badly broken.

[Alexander Larsson <alla lysator liu se>]

Quick test: Drag the window from the "buttons" test in testgtk outside the
screen and then in again. It spews GDK-Critical and the stock images look
bad.

The expose method of GtkImage just does a gdk_pixbuf_render_to_drawable_alpha()
on the whole image, if part of it overlaps the exposed area.
gdk_pixbuf_render_to_drawable_alpha() then calls
gdk_pixbuf_get_from_drawable(), which only work if the entire source
rectangle is inside the screen.

/ Alex