Alternative URL for still picture if camera is not reachable
**Describe the solution you'd like:**

I have implemented several cameras with the still-picture and stream URLs. Both work fine if camera is switched on and working. However - if there is an issue with the camera or I want to switch it off at certain times of the day to save energy the HomeKit app is usually showing no picture or a very old one. 

Additionally I save pictures every hour on a separate server (I switch all the cameras automatically on, save the pictures, and then switch the cameras off again. 

It would be excellent, if ffmpeg could detect, that if there is no picture coming from the switched off camera and then instead taking the last picture saved on the server.

Parameter could be like: "stillImageSourceAlternative": "/opt/saved_images/still_image.jpg",
