IsPendingKill replacement, fix for CalcBounds.

[Ryan-DowlingSoka]

IsValidChecked(this) replaces IsPendingKill()

FBox::ExpandBy() returns a new box.

     * Increase the bounding box volume.
     *
     * @param W The size to increase volume by.
     * @return A new bounding box increased in size.
     */
    FBox2D ExpandBy( const float W ) const
    {
        return FBox2D(Min - FVector2D(W, W), Max + FVector2D(W, W));
    }```