Fix flickering (#9012)

See https://zed.dev/channel/gpui-536

Fixes https://github.com/zed-industries/zed/issues/9010
Fixes https://github.com/zed-industries/zed/issues/8883
Fixes https://github.com/zed-industries/zed/issues/8640
Fixes https://github.com/zed-industries/zed/issues/8598
Fixes https://github.com/zed-industries/zed/issues/8579
Fixes https://github.com/zed-industries/zed/issues/8363
Fixes https://github.com/zed-industries/zed/issues/8207


### Problem

After transitioning Zed to GPUI 2, we started noticing that interacting
with the mouse on many UI elements would lead to a pretty annoying
flicker. The main issue with the old approach was that hover state was
calculated based on the previous frame. That is, when computing whether
a given element was hovered in the current frame, we would use
information about the same element in the previous frame.

However, inspecting the previous frame tells us very little about what
should be hovered in the current frame, as elements in the current frame
may have changed significantly.

### Solution

This pull request's main contribution is the introduction of a new
`after_layout` phase when redrawing the window. The key idea is that
we'll give every element a chance to register a hitbox (see
`ElementContext::insert_hitbox`) before painting anything. Then, during
the `paint` phase, elements can determine whether they're the topmost
and draw their hover state accordingly.

We are also removing the ability to give an arbitrary z-index to
elements. Instead, we will follow the much simpler painter's algorithm.
That is, an element that gets painted after will be drawn on top of an
element that got painted earlier. Elements can still escape their
current "stacking context" by using the new `ElementContext::defer_draw`
method (see `Overlay` for an example). Elements drawn using this method
will still be logically considered as being children of their original
parent (for keybinding, focus and cache invalidation purposes) but their
layout and paint passes will be deferred until the currently-drawn
element is done.

With these changes we also reworked geometry batching within the
`Scene`. The new approach uses an AABB tree to determine geometry
occlusion, which allows the GPU to render non-overlapping geometry in
parallel.

### Performance

Performance is slightly better than on `main` even though this new
approach is more correct and we're maintaining an extra data structure
(the AABB tree).


![before_after](https://github.com/zed-industries/zed/assets/482957/c8120b07-1dbd-4776-834a-d040e569a71e)

Release Notes:

- Fixed a bug that was causing popovers to flicker.

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>

crates/gpui/src/geometry.rs

@@ -828,6 +828,28 @@ where
             y: self.origin.y.clone() + self.size.height.clone().half(),
         }
     }
+
+    /// Calculates the half perimeter of a rectangle defined by the bounds.
+    ///
+    /// The half perimeter is calculated as the sum of the width and the height of the rectangle.
+    /// This method is generic over the type `T` which must implement the `Sub` trait to allow
+    /// calculation of the width and height from the bounds' origin and size, as well as the `Add` trait
+    /// to sum the width and height for the half perimeter.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use zed::{Bounds, Point, Size};
+    /// let bounds = Bounds {
+    ///     origin: Point { x: 0, y: 0 },
+    ///     size: Size { width: 10, height: 20 },
+    /// };
+    /// let half_perimeter = bounds.half_perimeter();
+    /// assert_eq!(half_perimeter, 30);
+    /// ```
+    pub fn half_perimeter(&self) -> T {
+        self.size.width.clone() + self.size.height.clone()
+    }
 }
 
 impl<T: Clone + Default + Debug + PartialOrd + Add<T, Output = T> + Sub<Output = T>> Bounds<T> {
@@ -1145,6 +1167,22 @@ where
     }
 }
 
+/// Checks if the bounds represent an empty area.
+///
+/// # Returns
+///
+/// Returns `true` if either the width or the height of the bounds is less than or equal to zero, indicating an empty area.
+impl<T: PartialOrd + Default + Debug + Clone> Bounds<T> {
+    /// Checks if the bounds represent an empty area.
+    ///
+    /// # Returns
+    ///
+    /// Returns `true` if either the width or the height of the bounds is less than or equal to zero, indicating an empty area.
+    pub fn is_empty(&self) -> bool {
+        self.size.width <= T::default() || self.size.height <= T::default()
+    }
+}
+
 impl Bounds<Pixels> {
     /// Scales the bounds by a given factor, typically used to adjust for display scaling.
     ///
@@ -2617,6 +2655,12 @@ pub trait Half {
     fn half(&self) -> Self;
 }
 
+impl Half for i32 {
+    fn half(&self) -> Self {
+        self / 2
+    }
+}
+
 impl Half for f32 {
     fn half(&self) -> Self {
         self / 2.