QImageCapture: Support queueing multiple still photos

At the time of writing, queueing of multiple captures in QImageCapture is explicitly disabled in the FFmpeg backend (QFFmpegImageCapture). This means if you try to call QImageCapture::capture() while another is pending, we return a capture-request ID of -1 and we signal an error.

Support queueing of still photos could be beneficial. We should look into how we can support this usecase, and how it might be done.

A related feature that we can consider is supporting burst-capture of still photos. Apple and Android platforms supports taking burst captures, where we only lock in auto-focus once, and then we take multiple photos in quick succession with the focus locked in. This is a popular feature in native camera apps on mobile devices.