Fixes #112689

#111384 introduced a warning when calling Camera2D::get_camera_transform() before the camera viewports had been set up in NOTIFICATION_ENTER_TREE.

This flagged a benign warning when using physics interpolation on Camera2D, because it inherits a standard reset from CanvasItem which is called before Camera2D NOTIFICATION_ENTER_TREE.

Here we use a simple approach to fix the warning, by adding a state flag of whether to accept resets, and set to true after setup, and set to false when exiting the tree.

Notes

• There's a number of ways of fixing this - we could alternatively just not spam the warning introduced in Fix incorrect transform calculation in Camera2D when using a custom viewport #111384. Although it's fairly easy to workaround here, and it is possible the warning might catch bugs.
• The post-warning and the previous version both work fine, as there is a secondary manual reset performed in Camera2D at the end of NOTIFICATION_ENTER_TREE (and a comment to that effect), so this warning is purely cosmetic.
• Not necessary in 3.x as we don't have the warning.

Discussion

As noted in the issue, this is caused by the CanvasItem NOTIFICATION_ENTER_TREE being called before the Camera2D, so the resets can't depend on any state set up in derived classes, which isn't ideal, but thus far this hasn't been a significant problem - most classes xforms are up to date by this point.

An alternative we sometimes use is to defer resets, but this has the potential to cause knock on bugs particularly with moving and standing starts with manual resets from script, so on balance it would be nice to avoid this bug potential and keep a local solution, at least for now. If we get any later similar issues we can re-evaluate this.