fix: center tutorial introduction paragraphs

tools/test_tutorial_intro.py

@@ -7,7 +7,7 @@ from test_cjk_layout import layout
 
 
 class IntroTests(unittest.TestCase):
-    def test_intro_paragraphs_are_left_aligned_without_orphan_punctuation(self):
+    def test_intro_paragraphs_are_centered_without_orphan_punctuation(self):
         mapping = json.loads(Path(__file__).with_name('tutorial_cn.json').read_text(encoding='utf-8'))
         intros = [v for k, v in mapping.items() if k.startswith('\fNow')]
         self.assertEqual(len(intros), 2)
@@ -15,7 +15,9 @@ class IntroTests(unittest.TestCase):
             for width in (360, 400, 480):
                 lines, _ = layout(intro, width=width, alignment=0)
                 for raw, x, y in lines:
-                    self.assertEqual(x, 0)
+                    text = raw.decode("gb2312")
+                    pixels = sum(5 if c == " " else 8 if ord(c) < 128 else 12 for c in text)
+                    self.assertEqual(x, (width - pixels) // 2)
                     self.assertNotIn(raw.decode('gb2312')[0], '，。！？、；：”）')
                 self.assertEqual(len(lines), 4)