Layout gutter hunk diff close button (X) better (#15178)

Closes https://github.com/zed-industries/zed/issues/15164

* Deleted hunk
Before:

![before_top](https://github.com/user-attachments/assets/27c72ee5-719f-4787-b222-4f597840936a)

After:

![after_top](https://github.com/user-attachments/assets/58095b21-698d-4778-8412-b680d5253e2c)

* Modified hunk
Before:

![before_down](https://github.com/user-attachments/assets/3fffb73b-7101-493c-b63b-15c3ccaf5362)

After:

![after_down](https://github.com/user-attachments/assets/f07a05ed-1260-4e2a-9388-c9cb93020d78)

* Added hunk
Before:

![before_mid](https://github.com/user-attachments/assets/0dd3f0f9-51e8-449a-bdd7-4c3ba5cd7791)

After:

![after_mid](https://github.com/user-attachments/assets/92f749bd-2d95-4650-8334-1d5c38c6aeb6)

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -594,7 +594,7 @@ pub struct EditorSnapshot {
 
 const GIT_BLAME_GUTTER_WIDTH_CHARS: f32 = 53.;
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Default, Debug, Clone, Copy)]
 pub struct GutterDimensions {
     pub left_padding: Pixels,
     pub right_padding: Pixels,
@@ -617,18 +617,6 @@ impl GutterDimensions {
     }
 }
 
-impl Default for GutterDimensions {
-    fn default() -> Self {
-        Self {
-            left_padding: Pixels::ZERO,
-            right_padding: Pixels::ZERO,
-            width: Pixels::ZERO,
-            margin: Pixels::ZERO,
-            git_blame_entries_width: None,
-        }
-    }
-}
-
 #[derive(Debug)]
 pub struct RemoteSelection {
     pub replica_id: ReplicaId,
@@ -5167,7 +5155,7 @@ impl Editor {
             }))
     }
 
-    fn render_close_hunk_diff_button(
+    fn close_hunk_diff_button(
         &self,
         hunk: HoveredHunk,
         row: DisplayRow,
@@ -11854,6 +11842,11 @@ impl Editor {
         let source_y = line_height * (source.row() - first_visible_line.row()).0 as f32;
         Some(gpui::Point::new(source_x, source_y))
     }
+
+    fn gutter_bounds(&self) -> Option<Bounds<Pixels>> {
+        let bounds = self.last_bounds?;
+        Some(element::gutter_bounds(bounds, self.gutter_dimensions))
+    }
 }
 
 fn hunks_for_selections(
@@ -12240,7 +12233,7 @@ impl EditorSnapshot {
         self.scroll_anchor.scroll_position(&self.display_snapshot)
     }
 
-    pub fn gutter_dimensions(
+    fn gutter_dimensions(
         &self,
         font_id: FontId,
         font_size: Pixels,