Add pivots support for Sprites directly

[RobotoSkunk]

I've been using Godot for a while and one thing that is missing with Godot's 2D system is support for sprites pivots.

I know that Godot's nodes already have offsets, but it's not the same as having a configurable pivot per sprite like in Unity and Game Maker Studio. It's pretty basic for 2D games and it's less painful for sprite animations.