Make Picture-in-Picture "mask" the window instead of drawing a region

Right now PiP works by drawing a rectangular region on the window, which has some issues:

- You only get one shot; if you mess up, you have to close PiP and start over
- The original window sticks around, meaning you have two copies of the content
- It's unclear what you're supposed to do when you hit the shortcut

Instead, we should switch to a corner grabber design that masks/clips the window down to the chosen PiP area, with an explicit "confirm" button to complete the area selection. Here is a sketch I did a while back:


And [here is a rough motion mockup on YouTube](https://www.youtube.com/watch?v=SDJYnZlqdn4).