week-grid: consider daylight when calculating event position

There are some weird cases around the globe where the timezone
changes in the middle of the day. In GMT-8:00, for example, the
DST changes at 2:00 am. This makes the timezone of thestart of
the week different from the event's timezone, and somehow the
GLib API doesn't take that into account when calculating the
diff between two dates.

Fix that by also considering different timezones when calculating
the event start and end positions.

https://bugzilla.gnome.org/show_bug.cgi?id=780079

src/views/gcal-week-grid.c

@@ -138,20 +138,26 @@ get_event_range (GcalWeekGrid *self,
 {
   GDateTime *week_start;
   GTimeSpan diff;
+  gboolean week_start_dst;
 
   if (!self->active_date)
     return;
 
   week_start = get_start_of_week (self->active_date);
+  week_start_dst = g_date_time_is_daylight_savings (week_start);
 
   if (start)
     {
       GDateTime *event_start;
+      gboolean event_start_dst;
 
       event_start = g_date_time_to_local (gcal_event_get_date_start (event));
+      event_start_dst = g_date_time_is_daylight_savings (event_start);
 
       diff = g_date_time_difference (event_start, week_start);
+
       *start = CLAMP (diff / G_TIME_SPAN_MINUTE, 0, MAX_MINUTES);
+      *start += 60 * (event_start_dst - week_start_dst);
 
       g_clear_pointer (&event_start, g_date_time_unref);
     }
@@ -160,11 +166,15 @@ get_event_range (GcalWeekGrid *self,
     {
 
       GDateTime *event_end;
+      gboolean event_end_dst;
 
       event_end = g_date_time_to_local (gcal_event_get_date_end (event));
+      event_end_dst = g_date_time_is_daylight_savings (event_end);
 
       diff = g_date_time_difference (event_end, week_start);
+
       *end = CLAMP (diff / G_TIME_SPAN_MINUTE, 0, MAX_MINUTES);
+      *end += 60 * (event_end_dst - week_start_dst);
 
       /*
        * XXX: it may happen that the event has the same start and end