Calendar export should filter on schedule items, not programs

Calendar export is invoked from a view that only shows _schedule items_ matching current filters:

The filters (in this case _show only favorites_ and _hide past programs_) are passed to the calendar export.

However, the export filters _program items_ with the same filters and then includes all schedule items of matching program items.

But a program item may have multiple schedule items, some of which may not pass the current filters even though their parent program items do:

In this case, I also get the Saturday run of this workshop in my calendar despite expecting to only get the Sunday one.