Fix dragging windows

This was broken in "Fix a type checker error" e62064a307 
where I just added `and self.offset_at_drag_start`.
The problem is that Offset defines __bool__ and are falsy when (0,0),
and the windows, despite starting out in the center, initially have an
offset of (0,0) FROM this center position.

README.md

@@ -141,7 +141,6 @@ cat file.ans
 - Large files can make the program very slow, as can magnifying the canvas.
 - The program sometimes crashes or freezes randomly, and there's no auto-save feature.
 - Saving/loading an ANSI file ends up with a white cell at the top left, which pushes the top row to the right. The top right cell gets pushed off and deleted.
-- Dragging windows is broken; it used to work, but now does nothing.
 - The Polygon/Curve tools aren't finalized when you switch tools.
 - Saved ANSI files are unnecessarily large, because they include escape sequences for every cell, even if the colors match the previous cell.
 

windows.py

@@ -184,7 +184,7 @@ class Window(Container):
 
     def on_mouse_move(self, event: events.MouseMove) -> None:
         """Called when the user moves the mouse."""
-        if self.mouse_at_drag_start and self.offset_at_drag_start:
+        if self.mouse_at_drag_start is not None and self.offset_at_drag_start is not None:
             self.styles.offset = (
                 self.offset_at_drag_start.x + event.screen_x - self.mouse_at_drag_start.x,
                 self.offset_at_drag_start.y + event.screen_y - self.mouse_at_drag_start.y,