Fix tooltips sometimes changing position each frame (#7304)

There was a bug in how we decide where to place a `Tooltip` (or other
`Popup`), which could lead to tooltips jumping around every frame,
especially if it changed size slightly.

The new code is simpler and bug-free.

crates/egui/src/containers/popup.rs

@@ -484,6 +484,7 @@ impl<'a> Popup<'a> {
             self.gap,
             expected_popup_size,
         )
+        .unwrap_or_default()
     }
 
     /// Show the popup.