Handle completion of todos in distant time zones

The Month view displays todos on their due dates in the local time zone;
a todo due on the 23rd at 06:00 Asia/Tokyo time is displayed on the
22nd if the local time zone is UTC or further west.

`IncidenceMonthItem::icons()` used the local date to calculate an
occurrence date for occurrences of recurring todos, without considering
time zones, to decide whether to draw a "completed" icon.  This can
fail.

Example:
- start 2022-09-22 06:00 Asia/Tokyo
- due   2022-09-23 07:00 Asia/Tokyo
- repeat daily, 3 times.  (Occurrences overlap.)
- use the Todo view to complete it once.

The Month view does not display any "completed" icons.

This patch uses a previously-computed offset between the occurrence and
the todo's start.