Change text color of selected text (#7691)

Selected text now gets the color of `visuals.selection.stroke.color`.
This means you can have inverted colors for selected text, like in the
new test:
<img width="154" height="46" alt="image"
src="https://github.com/user-attachments/assets/2666361d-d7e2-4d50-8e4d-2fcc128f1a81"
/>


It also means the color of selected text in labels matches that of the
text color of selected buttons.

crates/epaint/src/text/text_layout.rs

@@ -230,6 +230,7 @@ fn layout_section(
                 font_ascent: font_metrics.ascent,
                 uv_rect: glyph_alloc.uv_rect,
                 section_index,
+                first_vertex: 0, // filled in later
             });
 
             paragraph.cursor_x_px += glyph_alloc.advance_width_px;
@@ -531,6 +532,7 @@ fn replace_last_glyph_with_overflow_character(
                 font_ascent: font_metrics.ascent,
                 uv_rect: replacement_glyph_alloc.uv_rect,
                 section_index,
+                first_vertex: 0, // filled in later
             });
             return;
         }
@@ -748,7 +750,7 @@ fn tessellate_row(
     point_scale: PointScale,
     job: &LayoutJob,
     format_summary: &FormatSummary,
-    row: &Row,
+    row: &mut Row,
 ) -> RowVisuals {
     if row.glyphs.is_empty() {
         return Default::default();
@@ -843,8 +845,9 @@ fn add_row_backgrounds(point_scale: PointScale, job: &LayoutJob, row: &Row, mesh
     end_run(run_start.take(), last_rect.right());
 }
 
-fn tessellate_glyphs(point_scale: PointScale, job: &LayoutJob, row: &Row, mesh: &mut Mesh) {
-    for glyph in &row.glyphs {
+fn tessellate_glyphs(point_scale: PointScale, job: &LayoutJob, row: &mut Row, mesh: &mut Mesh) {
+    for glyph in &mut row.glyphs {
+        glyph.first_vertex = mesh.vertices.len() as u32;
         let uv_rect = glyph.uv_rect;
         if !uv_rect.is_nothing() {
             let mut left_top = glyph.pos + uv_rect.offset;

crates/epaint/src/text/text_layout_types.rs

@@ -701,6 +701,9 @@ pub struct Glyph {
     /// enable the paragraph-concat optimization path without having to
     /// adjust `section_index` when concatting.
     pub(crate) section_index: u32,
+
+    /// Which is our first vertex in [`RowVisuals::mesh`].
+    pub first_vertex: u32,
 }
 
 impl Glyph {