input: add a weston_pointer_clear_focus() helper function

Valgrind has shown that in at least one place (default_grab_pointer_focus)
we're testing uninitialized values coming out of weston_compositor_pick_view.

This is happening when default_grab_pointer_focus is called when there is
nothing on the view list, and during the first repaint when only the black
surface with no input region exists.

This patch adds a function to clear pointer focus and also set the sx,sy
co-ordinates to a sentinel value we shouldn't compute with.

Assertions are added to make sure any time pointer focus is set to NULL
these values are used.

weston_compositor_pick_view() now returns these values too.

Now the values are always initialized, even when no view exists, and
they're initialized in such a way that actually doing computation
with them should fail in an obvious way, but we can compare them
safely for equality.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>

desktop-shell/exposay.c

@@ -589,9 +589,7 @@ exposay_transition_active(struct desktop_shell *shell)
 	if (pointer) {
 		weston_pointer_start_grab(pointer,
 		                          &shell->exposay.grab_ptr);
-		weston_pointer_set_focus(pointer, NULL,
-				         pointer->x,
-					 pointer->y);
+		weston_pointer_clear_focus(pointer);
 	}
 	wl_list_for_each(shell_output, &shell->output_list, link) {
 		enum exposay_layout_state state;

desktop-shell/shell.c

@@ -3218,9 +3218,7 @@ popup_grab_focus(struct weston_pointer_grab *grab)
 	    wl_resource_get_client(view->surface->resource) == client) {
 		weston_pointer_set_focus(pointer, view, sx, sy);
 	} else {
-		weston_pointer_set_focus(pointer, NULL,
-					 wl_fixed_from_int(0),
-					 wl_fixed_from_int(0));
+		weston_pointer_clear_focus(pointer);
 	}
 }