Fix NaN TimeOfFlight at negative yOffset & low angle

[kostasvs]

TimeOfFlight currently returns NaN in some cases, particularly when firing angle is low and the target Y offset is negative.

The function has to solve a quadradic and it gets the solution for the positive root, but sometimes the correct solution is the one for negative root.

Example for testing:

var durationTest = ProjectileMath.TimeOfFlight(5, 5 * Mathf.Deg2Rad, -1.5f, 9.8);
Debug.Log($"angle: 5°, speed: 5m/s, offsetY: -1.5m, gravity: 9.8m/s², duration: {durationTest:F2}s");

Using current implementation, the TimeOfFlight returns NaN and the output is:

angle: 5°, speed: 5m/s, offsetY: -1.5m, gravity: 9,8m/s², duration: NaNs

Using this PR's implementation, TimeOfFlight returns correct result, and the output becomes:

angle: 5°, speed: 5m/s, offsetY: -1.5m, gravity: 9,8m/s², duration: 0,60s