Text: exact line spacing ends at the last line's descent

With exact line spacing (a:lnSpc/a:spcPts) PowerPoint puts the baseline at
80 % of the line pitch and ends the text at the last line's descent, not a
full pitch below it. We drew every line a full pitch tall, so table rows
whose cells use exact spacing grew taller than PowerPoint's (35.6 pt
instead of 33 pt for 12 pt type at 26.59 pt), pushing tables and text
boxes down and tripping shrink-on-overflow early.

Measured on PowerPoint's own PDF of a real deck: rows now come out at
33.5 pt against PowerPoint's 32.9-33.8 pt.

crates/text/src/tests.rs

@@ -54,6 +54,33 @@ fn shrink_on_overflow() {
     assert!(!l.overflow || l.font_scale <= 0.11);
 }
 
+#[test]
+fn exact_line_spacing_ends_at_the_last_descent() {
+    // 12 pt text at exactly 26.59 pt per line (a table cell PowerPoint sizes to 24.3 pt of text):
+    // the baseline at 80 % of the pitch, the text ending at the last line's descent.
+    let (p, mut sh) = setup("One\nTwo", 600.0, 300.0);
+    for para in &mut sh.text.as_mut().unwrap().paragraphs {
+        para.props.line_spacing = Some(Spacing::Pts(26.59));
+        para.props.bullet = Some(Bullet::None);
+        for r in &mut para.runs {
+            r.props.size = Some(12.0);
+        }
+    }
+    let l = lay(&p, &sh, 600.0, 300.0);
+    let first = &l.lines[0];
+    assert!((first.baseline - first.top - 26.59 * 0.8).abs() < 0.01, "{:?}", first.baseline - first.top);
+    // The text ends at the last line's descent: above its full pitch by 20 % of it less the
+    // descent (~2.3 pt for 12 pt type).
+    let short = (l.lines[1].bottom - l.lines[0].top) - l.content_height;
+    assert!(short > 1.0 && short < 26.59 * 0.2, "{short}");
+    // Proportional spacing keeps full pitches.
+    for para in &mut sh.text.as_mut().unwrap().paragraphs {
+        para.props.line_spacing = Some(Spacing::Pct(1.0));
+    }
+    let l = lay(&p, &sh, 600.0, 300.0);
+    assert!((l.content_height - (l.lines[1].bottom - l.lines[0].top)).abs() < 0.01);
+}
+
 #[test]
 fn caret_and_hit_roundtrip() {
     let (p, sh) = setup("Hello world", 600.0, 100.0);