Add FlxMatrix pooling

[MaybeMaru]

Same as with FlxPoint and FlxRect, FlxMatrix is a very commonly used class in HaxeFlixel for math calculations.
However, strangely, it is the only one of the group that lacks pooling support to reuse instances of FlxMatrix.

Adding the same kind of pooling that FlxPoint and FlxRect have would be extremely useful, both for developers and within HaxeFlixel itself, as it would reduce the number of instances and the memory overhead caused by constantly creating and destroying FlxMatrix objects.

[Geokureli]

Global pooling isn't worth it IMHO, all of the point handling methods with optional result args add a lot of complexity, and bugs due to human error is pretty common (both on flixel's end and the devs utilizing them). They require a few unit tests every time we add one, and we're not good about adding them all.

My actual goal is to deprecate and remove FlxCallbackPoint (by replacing FlxSpriteGroup), which would allow us to remove point pooling and inline the FlxBasePoint constructor, preventing any actual instantiation of a lot of FlxPoints. For most other cases raising the minimum supported haxe version to 4.3.7 will allow us to use a dedicated static local field for every method rather than recycling a new one each call.

Another issue with changing FlxMatrix to be pooled is that nothing is set up to use recycled matrices. So adding pool related fields we're just adding more memory to each matrix. If I were to add something like this, I would need to see benchmark tests proving the performance/memory benefits. My guess is that places that could benefit can likely use something simpler