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

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/main/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

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,

tests/egui_tests/tests/regression_tests.rs

@@ -1,8 +1,13 @@
+use std::sync::Arc;
+
+use egui::ScrollArea;
 use egui::accesskit::Role;
 use egui::epaint::Shape;
 use egui::style::ScrollAnimation;
+use egui::text::{LayoutJob, TextWrapping};
 use egui::{
-    Align, Color32, Image, Label, Layout, RichText, ScrollArea, Sense, TextWrapMode, include_image,
+    Align, Color32, FontFamily, FontId, Image, Label, Layout, RichText, Sense, TextBuffer,
+    TextFormat, TextWrapMode, Ui, include_image, vec2,
 };
 use egui_kittest::Harness;
 use egui_kittest::kittest::Queryable as _;
@@ -66,6 +71,53 @@ fn text_edit_rtl() {
     }
 }
 
+#[test]
+fn text_edit_halign() {
+    let mut harness = Harness::builder().with_size((212.0, 212.0)).build_ui(|ui| {
+        ui.spacing_mut().item_spacing = vec2(2.0, 2.0);
+
+        fn layouter(halign: Align) -> impl FnMut(&Ui, &dyn TextBuffer, f32) -> Arc<egui::Galley> {
+            move |ui: &egui::Ui, buf: &dyn egui::TextBuffer, wrap_width: f32| {
+                let mut job = LayoutJob {
+                    wrap: TextWrapping {
+                        max_rows: 4,
+                        max_width: wrap_width,
+                        ..Default::default()
+                    },
+                    halign,
+                    ..Default::default()
+                };
+                job.append(
+                    buf.as_str(),
+                    0.0,
+                    TextFormat::simple(FontId::new(13.0, FontFamily::Proportional), Color32::GRAY),
+                );
+                ui.fonts_mut(|f| f.layout_job(job))
+            }
+        }
+
+        for widget_alignment in [Align::Min, Align::Center, Align::Max] {
+            ui.horizontal(|ui| {
+                for text_alignment in [Align::LEFT, Align::Center, Align::RIGHT] {
+                    ui.add_sized(
+                        vec2(64.0, 64.0),
+                        egui::TextEdit::multiline(&mut format!(
+                            "{widget_alignment:?}\n+\n{text_alignment:?}",
+                        ))
+                        .layouter(&mut layouter(text_alignment))
+                        .vertical_align(widget_alignment)
+                        .horizontal_align(widget_alignment),
+                    );
+                }
+            });
+        }
+    });
+
+    harness.get_by_value("Center\n+\nCenter").focus();
+    harness.step();
+    harness.snapshot("text_edit_halign");
+}
+
 #[test]
 fn text_edit_delay() {
     let mut text = String::new();

tests/egui_tests/tests/snapshots/text_edit_halign.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:502607c803b884e4e1640d39c97b03b0a40df93c2da328f889168e386f837f36
+size 13261