v1.2.0: remove spurious \n from draw-rect

draw-rect wrote \n after each row's fill, including the last row at
the bottom of the frame. This caused a terminal scroll, shifting all
content up by 1 and leaving the last row blank (terminal default bg).
cursor-move-escape at the start of each iteration already repositions
the cursor — the \n was never needed.

org/modern-backend.org

@@ -817,7 +817,7 @@ color.
          (line (concatenate 'string
                  bg-esc
                  (make-string width :initial-element #\Space)
-                 reset (string #\Newline))))
+                 reset "")))
     (loop :for row :from 0 :below height :do
       (backend-write b (cursor-move-escape x (+ y row)))
       (backend-write b line))))