This commit fixes#758. I was having the same issue and looked into it a bit. I found
[this StackOverflow answer](https://stackoverflow.com/a/60585382) which described a
solution to the same problem. Rather than manually calculate the focus point, we can
get the PreviewView to do it for us. This fixes the issue because the PreviewView
factors in any scaling or resizing of the view on the screen, which we weren't doing
before. The only potential issue is that this needs to run on the UI thread
(which is what the `withContext` is doing), but I've tested it with frame processors
enabled and disabled, and have found no issues in either case.
