Compositor fix.

1. ChopsUpdate docstring was not correct.
2. ChopsUpdate.render_segments was generating longer sequences than
   necessary because it always cropped strips with a start of zero.

Author: paul-ollis

src/textual/_compositor.py

@@ -174,7 +174,7 @@ def __init__(
 
         Args:
             chops: A mapping of offsets to list of segments, per line.
-            crop: Region to restrict update to.
+            spans: Line spans to restrict update to.
             chop_ends: A list of the end offsets for each line
         """
         self.chops = chops
@@ -263,8 +263,8 @@ def render_segments(self, console: Console) -> str:
                     append(strip.render(console))
                     continue
 
-                strip = strip.crop(0, min(end, x2) - x)
-                append(move_to(x, y).segment.text)
+                strip = strip.crop(x1, min(end, x2) - x)
+                append(move_to(x1, y).segment.text)
                 append(strip.render(console))
 
             if y != last_y: