Allow focusing only on the viewport, ignore outside touches

[JacekJagosz]

This is kind of a feature request and kind of a bug report. If you have chosen photo proportions not the same as your screen e.g. 4:3 you have black bars on each side. But you still can touch those black bars and the camera will try to focus there. But it will try to focus there indefinitely, preventing me from taking the photo.
I find that using the advanced camera on both X10 and 10 III with really small borders I often end up clicking accidentally at the corner, and the focus won't change, but it is now impossible to take the photo.
Could you please when taking the new coordinate for focusing check if it is within the viewport and ignore it if it isn't? It would prevent a lot of accidental clicks, and make the camera experience more reliable.