[librsvg: 3/15] Add a test to ensure we are using the Ahem font correctly

[Marge Bot <marge-bot src gnome org>]

commit ff1c99b7adc6a62547df77524e2ea9aa3b5b85ab
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Oct 8 17:43:07 2021 -0500

    Add a test to ensure we are using the Ahem font correctly
    
    Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/613>

 tests/src/primitives.rs | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
---
diff --git a/tests/src/primitives.rs b/tests/src/primitives.rs
index e1564bf8..638f00d2 100644
--- a/tests/src/primitives.rs
+++ b/tests/src/primitives.rs
@@ -400,3 +400,27 @@ test_compare_render_output!(
   <rect x="0" y="0" width="20" height="20" fill="black" transform="translate(-10 -20) scale(2) rotate(45)"/>
 </svg>"##,
 );
+
+// From https://www.w3.org/Style/CSS/Test/Fonts/Ahem/
+//
+//   > The Ahem font was developed by Todd Fahrner and Myles C. Maxfield to
+//   > help test writers develop predictable tests. The units per em is 1000,
+//   > the ascent is 800, and the descent is 200, thereby making the em
+//   > square exactly square. The glyphs for most characters is simply a box
+//   > which fills this square. The codepoints mapped to this full square
+//   > with a full advance are the following ranges:
+//
+// So, ascent is 4/5 of the font-size, descent is 1/5.  Mind the positions below.
+test_compare_render_output!(
+    ahem_font,
+    500,
+    500,
+    br##"<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg"; width="500" height="500">
+  <text style="font: 50px Ahem;" x="50" y="50" fill="black">abcde</text>
+</svg>"##,
+    br##"<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg"; width="500" height="500">
+  <rect x="50" y="10" width="250" height="50" fill="black"/>
+</svg>"##,
+);