Revert "gpui: Unify the index_for_x methods (#42162)" (#42505)

This reverts commit 082b80ec89.

This broke clicking, e.g. in snippets like

```rs
let x = vec![
    1, 2, //
    3,
];
```

clicking between `2` and `,` is quite off now.

Release Notes:

- N/A

crates/editor/src/display_map.rs

@@ -1097,7 +1097,7 @@ impl DisplaySnapshot {
         details: &TextLayoutDetails,
     ) -> u32 {
         let layout_line = self.layout_row(display_row, details);
-        layout_line.index_for_x(x) as u32
+        layout_line.closest_index_for_x(x) as u32
     }
 
     pub fn grapheme_at(&self, mut point: DisplayPoint) -> Option<SharedString> {

crates/editor/src/element.rs

@@ -8668,7 +8668,7 @@ impl LineWithInvisibles {
                     let fragment_end_x = fragment_start_x + shaped_line.width;
                     if x < fragment_end_x {
                         return Some(
-                            fragment_start_index + shaped_line.index_for_x(x - fragment_start_x),
+                            fragment_start_index + shaped_line.index_for_x(x - fragment_start_x)?,
                         );
                     }
                     fragment_start_x = fragment_end_x;

crates/editor/src/selections_collection.rs

@@ -372,7 +372,7 @@ impl SelectionsCollection {
         let is_empty = positions.start == positions.end;
         let line_len = display_map.line_len(row);
         let line = display_map.layout_row(row, text_layout_details);
-        let start_col = line.index_for_x(positions.start) as u32;
+        let start_col = line.closest_index_for_x(positions.start) as u32;
 
         let (start, end) = if is_empty {
             let point = DisplayPoint::new(row, std::cmp::min(start_col, line_len));
@@ -382,7 +382,7 @@ impl SelectionsCollection {
                 return None;
             }
             let start = DisplayPoint::new(row, start_col);
-            let end_col = line.index_for_x(positions.end) as u32;
+            let end_col = line.closest_index_for_x(positions.end) as u32;
             let end = DisplayPoint::new(row, end_col);
             (start, end)
         };