Return of a registration point for special purposes

`b().reg([ 20, 55 ])` allows to change it.

How it's related to pivot point in player:

* **NB** it's common to name _pivot_ a point used for rotation tween and _registration point_ a point used for position and translation tweens in most of the tools, but it's **not** the case in the player, at least for now. See notes below on what exactly _pivot_ and _reg-point_ currently mean in player;
* pivot is specified relatively to the dimensions (even if they are updated dynamically) of an element, while registration point is specified in exact pixel values;
* pivot point and registration point are applied for all of the shapes one after another on every cycle of animation (pivot is first);
* default pivot is `[0.5, 0.5]` and default registration point is `[0, 0]`;
* it is recommended to use pivot in most cases and only when you are required to use exact pixels or do not know the dimensions of the object, then feel free to use registration point.