This request asks the compositor to start a drag and drop operation on behalf of the client. The source argument is the data source that provides the data for the eventual data transfer. The origin surface is the surface where the drag originates and the client must have an active implicit grab that matches the timestamp. The icon surface is an optional (can be nil) surface that provides an icon to be moved around with the cursor. Initially, the top-left corner of the icon surface is placed at the cursor hotspot, but subsequent surface.attach request can move the relative position.