Viewer crashes when dragging off screen

[elliottslaughter]

I noticed while playing around today that if you drag the mouse off the screen, the viewer will crash at the following line:

prof-viewer/src/app.rs

Line 2232 in 7b9918d

let current = response.interact_pointer_pos().unwrap();

The trivial fix is not desirable because it would (presumably) cause the drag region to disappear entirely. Therefore, we might need to preserve the last known drag location so that we can keep rendering while the drag is off-screen.