input: Don't test keyboard/pointer/touch pointers

Keyboards and pointers aren't freed when devices are removed, so we should
really be testing keyboard_device_count and pointer_device_count in most
cases, not the actual pointers. Otherwise we end up with different
behaviour after removing a device than we had before it was inserted.

This commit renames the touch/keyboard/pointer pointers and adds helper
functions to get them that hide this complexity and return NULL when
*_device_count is 0.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jonas Ã…dahl <jadahl@gmail.com>
diff --git a/xwayland/dnd.c b/xwayland/dnd.c
index eb9c5dd..a036b30 100644
--- a/xwayland/dnd.c
+++ b/xwayland/dnd.c
@@ -154,6 +154,7 @@
 {
 	struct dnd_data_source *source;
 	struct weston_seat *seat = weston_wm_pick_seat(wm);
+	struct weston_pointer *pointer = weston_seat_get_pointer(seat);
 	char **p;
 	const char *name;
 	uint32_t *types;
@@ -213,7 +214,7 @@
 	}
 
 	free(reply);
-	weston_pointer_start_drag(seat->pointer, &source->base, NULL, NULL);
+	weston_pointer_start_drag(pointer, &source->base, NULL, NULL);
 }
 
 int