Fix distanceTo regression in FlxPoint

[oscarcederberg]

Fix regression introduced in 6.0.0 where a multiply in distanceSquaredTo was changed to an add, meaning all distanceTo/dist-functions got broken.

Added tests for all the affected functions as well.

In case a minimal setup to test the issue is needed:

package;

import flixel.math.FlxPoint;
import flixel.FlxG;
import flixel.FlxSprite;
import flixel.FlxState;
import flixel.text.FlxText;
import flixel.util.FlxColor;

class PlayState extends FlxState
{
    var player = new FlxSprite(0, 128);
    var enemy = new FlxSprite (0, 0);
    var text = new FlxText(0, 0, 0, "", 16);

    override public function create()
    {
        super.create();
        player.makeGraphic(32, 32, FlxColor.WHITE);
        enemy.makeGraphic(32, 32, FlxColor.RED);

        player.screenCenter(X);
        enemy.screenCenter(XY);

        add(text);
        add(player);
        add(enemy);
    }

    override public function update(elapsed:Float)
    {
        var up:Bool = FlxG.keys.pressed.UP;
        var down:Bool = FlxG.keys.pressed.DOWN;
        var left:Bool = FlxG.keys.pressed.LEFT;
        var right:Bool = FlxG.keys.pressed.RIGHT;

        if (up && down) {
            up = down = false;
        }
        if (left && right) {
            left = right = false;
        }

        var dx = 0, dy = 0;
        dx = left ? -1 : dx;
        dx = right ? 1 : dx;
        dy = up ? -1 : dy;
        dy = down ? 1 : dy;

        var velocity = 250 * new FlxPoint(dx, dy).normalize();
        player.velocity.set(velocity.x, velocity.y);

        text.text = 'distanceTo: ${player.getPosition().distanceTo(enemy.getPosition())}
        distanceSquaredTo: ${player.getPosition().distanceSquaredTo(enemy.getPosition())}
        dist: ${player.getPosition().dist(enemy.getPosition())}
        distSquared: ${player.getPosition().distSquared(enemy.getPosition())}
        ';

        super.update(elapsed);
    }
}

Note how distanceTo/dist reports NaN or weird values depending on if the player is above or below the enemy and that distanceSquaredTo/distSquared can get negative values without the fix.

[Geokureli]

Thanks!