Camera flash not working on iOS

When trying to use declarative-camera with FFmpeg backend on IOS, flash is never shown as supported, the controls for modifying flash do not appear. In the FFmpeg backend source code, the values for flash-support are never updated. This feature was previously supported on the iOS native Darwin backend, and is also supported on our Android build.

Steps to reproduce:

1. Use an iOS device with a physical flash, such as an iPhone or an iPad Pro.
2. Open declarative-camera on iOS, make sure to use FFmpeg backend.
3. Look at the camera preview, the relevant button controls are not there.

What do you expect to happen:
I expect the declarative-camera example to show the flash controls, like they do on Android. I expect the torch button to turn on the flash constantly like it does on Android. I expect the flash-button to enable auto-flashing when I do a photo capture, like it does on Android.

What actually happens:
When I run through the steps, the associated controls are not there.

Other notes:
If we modify the file 'qtmultimedia/src/plugins/ffmpeg/qavfcamera.mm', line 150, to include the lines

if (active)
        updateCameraConfiguration();

Then the values for whether flash mode is supported will be updated in backend. This will allow the controls to show up, but the flash-functionality still doesn't work even if you try to do a camera capture. I suspect we're not setting the correct values in AVCaptureDevice when flash-mode is set. There's also the option of migrating our existing AVCaptureStillImageOutput code (deprecated API) to the newer AVPhotoCaptureOutput API.