Deprecate screenCenter(), add gameCenter() and viewCenter()

[richTrash21]

Closes #3309

[Geokureli]

this is all good but I'm not gonna add this to the 5.9.0 release as we need to update all the documentation, demos and snippets, first

[Geokureli]

Actually we need to also update the unit tests, first.
https://github.com/HaxeFlixel/flixel/blob/dev/tests/unit/src/flixel/FlxObjectTest.hx#L344-L371

Should also add a separate test for viewCenter

[richTrash21]

this is all good but I'm not gonna add this to the 5.9.0 release as we need to update all the documentation, demos and snippets, first

oh ok, so i guess it will be merged in 6.0.0?

[Geokureli]

changed my mind, I need to edit a bunch of demos for 5.9.0 already, so this is now on 5.9.0

[Geokureli]

Sorry, changed my mind again, I'm not sure I'm happy with these names, and I think there's other issues, involved here.
gameCenter isn't very intuitive, and I feel like viewCenter should use the graphical midpoint, not the hitbox midpoint, or maybe there should be two, and maybe this is also true for gameCenter?

I'm leaning now, towards FlxG.center(sprite) and FlxG.centerHitBox(obj) and maybe myCamera.center(sprite) or something. I may shelf this for a bit and think about it.

For now, screenCenter is used a ton, and deprecating it at the last second is gonna impact a lot of projects, when most people are using it just to center something on a ui screen, and there's already plenty of deprecation in 5.9.0. So this will either go to 5.10.0 or 6.0. in the future we're planning on having colliders and images be a component of FlxObjects and FlxSprites, so maybe it makes sense to do camera.center(sprite.image) or FlxG.center(obj.collider) when that happens

[NeeEoo]

I would also kinda like a sprite.centerTo(otherSprite) or different function names

to center a sprite on top of another sprite

[Geokureli]

I would also kinda like a sprite.centerTo(otherSprite) or different function names

to center a sprite on top of another sprite

I've been thinking about that too, but that might be better as a static extension, and it has the same issue of confusing collider width/height with graphic width/height

For now I recommend doing the static extension yourself

import flixel.FlxObject;
import flixel.utils.FlxAxes;

class AlignTools
{
    static inline function centerOn<T:FlxObject>(objA:T, objB:FlxObject, axes:FlxAxes):T
    {
        if (axes.x) objA.x = objB.x + (objB.width - objA.width) / 2;
        if (axes.y) objA.y = objB.y + (objB.height - objA.height) / 2;
        return objA;
    }
}