title: allow eventOverlap for background events only in fullcalendar
body: <p>Is there a way to avoid event overlapping;
  like the eventOverlap: false inside the fullcalendar config,
  but on other hand allow overlap for background events?</p>
  
  <p>I want to render some events as background events into my calendar, just as info (that there are already some events in other calendars) but allow me to create, move or resize my new event on top.</p>
  
  <p>But all other events are not allowed to overlap inside this calendar.</p>
  
  <p>I just try this, without success:</p>
  
  <pre><code>   calendar:{
        editable: true,
        contentHeight: 'auto',
        theme: true,
        firstDay: 1,
        eventOverlap: false,
        nowIndicator: true,
        allDaySlot: false,
        slotDuration: '01:00:00',
        snapDuration: '00:00:01',
        axisFormat: 'HH:mm',
        timeFormat: 'HH:mm',
        timezone: 'local',
        views: {
          listThreeDay: {
            type: 'list',
            duration: { days: 3 },
            buttonText: 'list 3 day'
          },
          listOneWeek: {
            type: 'list',
            duration: { weeks: 1 },
            buttonText: 'list week'
          }
        },
        businessHours: {
          start: '06:00',         // a start time (6am in this example)
          end: '18:00',           // an end time (6pm in this example)
          dow: [ 1, 2, 3, 4, 5 ]  // days of week (here monday till friday)
        },
        events: [
          {
            start: '00:00:00+02:00',
            end: '08:00:00+02:00',
            color: 'red',
            rendering: 'background',
            dow: [1],
            overlap: true,
          }
        ],
       ...
  </code></pre>
  
  <p>Following picture shows what I need:</p>
  
  <ul>
  <li>background events (the red) </li>
  <li>normal events (blue) overlapping background events </li>
  <li>normal events (blue) do not overlap on other normal events</li>
  </ul>
  
  <p><a href="https://i.stack.imgur.com/EAIrl.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/EAIrl.png" alt="enter image description here"></a></p>