Fix godotengine#89578: incorrect work of look_at function

When using "Compatibility" rendering method, "scale_mode" set an integer and viewport height greater than the window height, viewport becomes misaligned with window leading to a rendering offset.
This causes the engine's mouse position to no longer match the real mouse position.
To fix this issue, the viewport, which was previously render-wise attached to the window's bottom-left corner was changed to be attached to the window's center.

scene/main/window.cpp

@@ -1146,10 +1146,8 @@ void Window::_update_viewport_size() {
  offset.x = Math::round(margin.x * viewport_size.y / screen_size.y);
  }
 
- if (screen_size.y < video_mode.y) {
- margin.y = Math::round((video_mode.y - screen_size.y) / 2.0);
- offset.y = Math::round(margin.y * viewport_size.x / screen_size.x);
- }
+ margin.y = Math::round((video_mode.y - screen_size.y) / 2.0);
+ offset.y = Math::round(margin.y * viewport_size.x / screen_size.x);
 
  switch (content_scale_mode) {
  case CONTENT_SCALE_MODE_DISABLED: {