Changed doc of GdkPixbufAnimation about size management.
Changed doc of GdkPixbufAnimation about size management.

Animation frames can be a different size managing this should be done
through GdkPixbufAnimation.

GNOME/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,GNOME/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,djdeath/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,djdeath/gdk-pixbuf,Distrotech/gdk-pixbuf,djdeath/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,djdeath/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf