Update area struct to allow force resizing (#7114)

This is a really small PR so I am skipping the issue (based on
contributing.md). This change adds an optional field and thus non
breaking for the API.

I ran into an issue during my development of an alerts manager widget
([see PR](https://github.com/blackberryfloat/egui_widget_ext/pull/2))
where I needed a scrollable overlay that did not block clicking areas of
a parent widget when my alerts did not take up the entire parent. To
achieve this I detect the sizing pass via the invisible flag and only
render the alerts content and then on the next pass I add the scroll bar
in around the alert content. Whenever the alert content changed though I
would need to create a new Area with a new id to get proper sizing. That
is a memory leak so I wanted to reset the size state to trigger a sizing
pass. Memory is rightfully protected enough that the path to remove
memory was dropped and I just added a hook to set a resize flag.

I am sure there are better ways but this is what made sense to me.
Looking forward to thoughts.

~~Logistics wise, I have proposed it as a patch because I was based off
0.31.1 for testing. I was also thinking it could be released quickly. I
am happy to cherry pick onto main after. If that is not allowed I can
rebase to main and pull against that.~~ (rebased on main)

---------

Co-authored-by: Wesley Murray <murraywj97@gmail.com>

crates/egui/src/containers/area.rs

@@ -121,6 +121,7 @@ pub struct Area {
     new_pos: Option<Pos2>,
     fade_in: bool,
     layout: Layout,
+    sizing_pass: bool,
 }
 
 impl WidgetWithState for Area {
@@ -147,6 +148,7 @@ impl Area {
             anchor: None,
             fade_in: true,
             layout: Layout::default(),
+            sizing_pass: false,
         }
     }
 
@@ -357,6 +359,27 @@ impl Area {
         self.layout = layout;
         self
     }
+
+    /// While true, a sizing pass will be done. This means the area will be invisible
+    /// and the contents will be laid out to estimate the proper containing size of the area.
+    /// If false, there will be no change to the default area behavior. This is useful if the
+    /// area contents area dynamic and you need to need to make sure the area adjusts its size
+    /// accordingly.
+    ///
+    /// This should only be set to true during the specific frames you want force a sizing pass.
+    /// Do NOT hard-code this as `.sizing_pass(true)`, as it will cause the area to never be
+    /// visible.
+    ///
+    /// # Arguments
+    /// - resize: If true, the area will be resized to fit its contents. False will keep the
+    ///   default area resizing behavior.
+    ///
+    /// Default: `false`.
+    #[inline]
+    pub fn sizing_pass(mut self, resize: bool) -> Self {
+        self.sizing_pass = resize;
+        self
+    }
 }
 
 pub(crate) struct Prepared {
@@ -410,6 +433,7 @@ impl Area {
             constrain_rect,
             fade_in,
             layout,
+            sizing_pass: force_sizing_pass,
         } = self;
 
         let constrain_rect = constrain_rect.unwrap_or_else(|| ctx.screen_rect());
@@ -425,6 +449,10 @@ impl Area {
             interactable,
             last_became_visible_at: None,
         });
+        if force_sizing_pass {
+            sizing_pass = true;
+            state.size = None;
+        }
         state.pivot = pivot;
         state.interactable = interactable;
         if let Some(new_pos) = new_pos {