pointer: Don't fizzle out surface changes too soon

During a DND grab, pointer->focus_surface is NULL, since the wl_pointer
doesn't have any focused surface (it's in drag mode). In this case, the
drag interface has control of the focus, and when dragging into a NULL
surface, drag_grab_focus won't get called, properly detaching it from
the previous surface.

Let the interface->focus implementation do the fizzling out.

In the future, we should split out wl_pointer's implementation
(pointer->focus_surface) from the Wayland side of the generic pointer
wrapper (pointer->current) and use our event routing system to determine
or similar whether it should go to wl_pointer or wl_data_device.
pointer: Fix the behavior of the pointer under DND

When grabbing with DND, we need to leave the pointer alone and
under the client's control. The code here was a bit messy before about
when it unset the window cursor -- it did it whenever there was no
current surface after repicking, which is a bit wrong, since it will
fire during a drag grab.

Move the check for this to update_cursor_surface, which is our standard
"sync" API for this, and then call update_cursor_surface after we set
the focus.
Merge branch 'master' of https://github.com/wolfv/mutter
compiles, doesnt owrk