Fix soft hyphens not being preserved when round tripping text through the terminal

Also roundtrip all characters in the Cf category.

Characters with the DI (Default Ignorable) property are now
preserved but not rendered and treated as zero-width
as per the unicode standard.
See https://www.unicode.org/faq/unsup_char.html

kitty_tests/datatypes.py

@@ -369,6 +369,7 @@ class TestDataTypes(BaseTest):
         self.ae(wcswidth('\U0001F1E6a\U0001F1E8a'), 6)
         self.ae(wcswidth('\U0001F1E6\U0001F1E8a'), 3)
         self.ae(wcswidth('\U0001F1E6\U0001F1E8\U0001F1E6'), 4)
+        self.ae(wcswidth('a\u00adb'), 2)
         # Regional indicator symbols (unicode flags) are defined as having
         # Emoji_Presentation so must have width 2
         self.ae(tuple(map(w, '\U0001f1ee\U0001f1f3')), (2, 2))

kitty_tests/screen.py

@@ -513,6 +513,15 @@ class TestScreen(BaseTest):
         self.ae(s.text_for_selection(), expected)
         s.scroll(2, True)
         self.ae(s.text_for_selection(), expected)
+        s.reset()
+
+    def test_soft_hyphen(self):
+        s = self.create_screen()
+        s.draw('a\u00adb')
+        self.ae(s.cursor.x, 2)
+        s.start_selection(0, 0)
+        s.update_selection(2, 0)
+        self.ae(s.text_for_selection(), ('a\u00adb',))
 
     def test_variation_selectors(self):
         s = self.create_screen()