Dont copy prompt start bits to multiple lines when resizing the screen

This makes it unnecessary to modify the prompt marking protocol to
distinguish consecutive prompts without output in between

kitty_tests/screen.py

@@ -955,7 +955,7 @@ class TestScreen(BaseTest):
             s.carriage_return(), s.index()
             s.draw(str(i))
         self.assertTrue(s.scroll_to_prompt())
-        self.ae(str(s.visual_line(0)), '$ 0')
+        self.ae(str(s.visual_line(0)), '$ 1')
 
         def lco():
             a = []