Fix panel drag leaking through overlay (#10035)

Closes #10017. While reworking the `overlay` element in #9911, I did not
realize that all overlay elements called `defer_draw` with a priority of
`1`.

/cc @as-cii 

Not including release notes, since it was only present in nightly.

Release Notes:

- N/A

crates/gpui/src/elements/deferred.rs

@@ -15,6 +15,16 @@ pub struct Deferred {
     priority: usize,
 }
 
+impl Deferred {
+    /// Sets the `priority` value of the `deferred` element, which
+    /// determines the drawing order relative to other deferred elements,
+    /// with higher values being drawn on top.
+    pub fn with_priority(mut self, priority: usize) -> Self {
+        self.priority = priority;
+        self
+    }
+}
+
 impl Element for Deferred {
     type BeforeLayout = ();
     type AfterLayout = ();