setAlignment support custom Alignment, like Alignment(-0.5, -0.5). (#424)

Author: lunxinfeng

lib/src/utils/calc_window_position.dart

@@ -79,6 +79,13 @@ Future<Offset> calcWindowPosition(
       visibleStartX + visibleWidth - windowSize.width,
       visibleStartY + (visibleHeight - windowSize.height),
     );
+  } else {
+    final left = (visibleWidth - windowSize.width) / 2 + alignment.x * ((visibleWidth - windowSize.width) / 2);
+    final top = (visibleHeight - windowSize.height) / 2 + alignment.y * ((visibleHeight - windowSize.height) / 2);
+    position = Offset(
+      visibleStartX + left,
+      visibleStartY + top,
+    );
   }
   return position;
 }