Fix spice_display_get_pixbuf() with offset area
Fix spice_display_get_pixbuf() with offset area

Fix screenshot of secondary displays, with an area position != (0,0).

This has never been working correctly since the surface display "area"
was introducted in:

commit e3bb7b1cfd162fcb8943e9d582dab43eeec6ce41
Author: Marc-André Lureau <marcandre.lureau@redhat.com>
Date:   Tue Jun 12 19:24:47 2012 +0200

    display: learn to restrict display to an area

https://bugzilla.redhat.com/show_bug.cgi?id=1029761

freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk