gpui: Disable rounding in the layout engine (#31836)

Rounding broke (among other things, probably) pixel-perfect image
rendering with non-power-of-two scaling factor.

An example which reproduces the problem can be found
[here](https://github.com/WaffleLapkin/gpui_taffy_rounding_whyyyyy).

How it looks with `gpui` from `main`:
![2025-05-31
11:34:25+CEST](https://github.com/user-attachments/assets/2cb19312-6ba6-4e80-8072-f89ddedff77b)

How it looks with this patch:
![2025-05-31
11:35:28+CEST](https://github.com/user-attachments/assets/114b52a9-58c0-4600-871c-a20eceb7179e)

Both screenshots are made on kde+wayland with magnification using kde's
built-in magnification (`Meta`+`+`, `Meta`+`-`). Note that screenshot
apps have a high chance of lying 🙃

The image itself is 400 by 300 pixels of red/green checkerboard pattern
made specifically to exaggerate scaling issues.

Release Notes:

- N/A

crates/editor/src/editor_tests.rs

@@ -15440,7 +15440,7 @@ async fn test_completions_default_resolve_data_handling(cx: &mut TestAppContext)
     // Completions that have already been resolved are skipped.
     assert_eq!(
         *resolved_items.lock(),
-        items[items.len() - 16..items.len() - 4]
+        items[items.len() - 17..items.len() - 4]
             .iter()
             .cloned()
             .map(|mut item| {

crates/gpui/src/taffy.rs

@@ -28,8 +28,10 @@ const EXPECT_MESSAGE: &str = "we should avoid taffy layout errors by constructio
 
 impl TaffyLayoutEngine {
     pub fn new() -> Self {
+        let mut taffy = TaffyTree::new();
+        taffy.disable_rounding();
         TaffyLayoutEngine {
-            taffy: TaffyTree::new(),
+            taffy,
             absolute_layout_bounds: FxHashMap::default(),
             computed_layouts: FxHashSet::default(),
         }