Enables every combination of TextEdit and LayoutJob alignments (#7831)

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This is a fix/improvement that makes all kinds of alignments work in
`TextEdit` when a custom `LayoutJob` with halign is used.


I used the simplest approach possible to avoid unwanted bugs as I wasn't
sure what's safe to change, but there's potentially better ways to
achieve this. In particular, I'm not sure I fully understand the
rationale behind aligning rows in a `Galley` based on the latter's
leftmost border, considering the size is what's ultimately used in
widgets (in `TextEdit` at least).

Regardless, here's a demo of this PR:


https://github.com/user-attachments/assets/5d9801d7-73af-4576-80c5-47f169700462


* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/widgets/text_edit/builder.rs

@@ -820,7 +820,11 @@ impl TextEdit<'_> {
                 paint_text_selection(&mut galley, ui.visuals(), &cursor_range, None);
             }
 
-            painter.galley(galley_pos, Arc::clone(&galley), text_color);
+            painter.galley(
+                galley_pos - vec2(galley.rect.left(), 0.0),
+                Arc::clone(&galley),
+                text_color,
+            );
 
             if has_focus && let Some(cursor_range) = state.cursor.range(&galley) {
                 let primary_cursor_rect = cursor_rect(&galley, &cursor_range.primary, row_height)

crates/epaint/src/text/text_layout_types.rs

@@ -1047,7 +1047,7 @@ impl Galley {
             return self.end_pos();
         };
 
-        let x = row.x_offset(layout_cursor.column);
+        let x = row.x_offset(layout_cursor.column) + row.pos.x - self.rect.left();
         Rect::from_min_max(pos2(x, row.min_y()), pos2(x, row.max_y()))
     }
 
@@ -1092,7 +1092,7 @@ impl Galley {
             if is_pos_within_row || y_dist < best_y_dist {
                 best_y_dist = y_dist;
                 // char_at is `Row` not `PlacedRow` relative which means we have to subtract the pos.
-                let column = row.char_at(pos.x - row.pos.x);
+                let column = row.char_at(pos.x - row.pos.x + self.rect.left());
                 let prefer_next_row = column < row.char_count_excluding_newline();
                 cursor = CCursor {
                     index: ccursor_index + column,