[client] wayland: use locked pointer to implement capture mode

This prevents the host cursor from moving into another window in capture
mode, solving the problem of input going to an overlapping window in
capture mode, and also preventing loss of focus with focus_follows_mouse.

client/displayservers/Wayland/wayland.h

@@ -126,6 +126,7 @@ struct WaylandDSState
   struct zwp_pointer_constraints_v1 * pointerConstraints;
   struct zwp_relative_pointer_v1 * relativePointer;
   struct zwp_confined_pointer_v1 * confinedPointer;
+  struct zwp_locked_pointer_v1 * lockedPointer;
   bool showPointer;
   uint32_t pointerEnterSerial;