Hide ejection angle labels when they are behind you

[Clayell]

Without this, the labels will appear in front of the camera again, which looks very strange. Found this bug when working on the LTP remake.

Tested to be working in-game

[Clayell]

Additional code to make the label disappear when far away:

        bool cameraNear = PlanetariumCamera.fetch.Distance < 100000d * (BodyOrigin.Radius / 6371000d); // 6371000 is earth radius

        if (periapsis.z > 0 && cameraNear)
        {
            GUI.Label(
                new Rect(
                    periapsis.x - 50,
                    Screen.height - periapsis.y - 15,
                    100, 30),
                $"Burn position", _styleLabelEnd);
        }

        if (asymptote.z > 0 && cameraNear)
        {
            GUI.Label(
                new Rect(
                    asymptote.x - 50,
                    Screen.height - asymptote.y - 15,
                    100, 30),
                "Escape direction", _styleLabelTarget);
        }