[PATCH wayland] Add a relative_grab request to the wl_shell_surface interface

[Vincent Povirk]

> Yup, good points here.  I think we could reuse the wl_pointer
> interface, but just send relative motion in the motion event and it
> should go throught the wl_seat capability feature as Daniel describes.

With this approach, is it going to be possible to know both the
relative motion for an event and the absolute position the cursor
moved to? SDL reports both relative and absolute cursor positions in a
single event, and on Windows I believe this can include relative
motion even when the cursor is clipped by a screen edge.

(And anyway, I'm pretty sure Wine needs both relative and absolute
coordinates to implement the mouse input API's correctly.)