[ts][pixi-v7][pixi-v8] Provide a way to define a fixed bounds

[davidetan]

Our runtimes usually offer a way to define a bounds.
In spine-pixi-v8 the game object bounds work like this:

1. If some bounding box attachments are active, the bounds will follow them
2. Otherwise the bounds is dynamically calculated each frame

Solution 1 is rarely used. Moreover, if someone wants to use bounding box for other reasons, they will affect inevitably the game object bounds.

Solution 2 does not allow to easily size the game object. Moreover, it recalculates the bounds each frame.

Our runtimes usually offer to define a fixed bounds based on:

• An animation and, optionally, a skin
• An AABB rectangle

Once defined, the bounds does not change, unless the user redifine them.

We should allow users to define bounds as in other runtimes.

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In spine-pixi-v7 the behaviour is similar, but the implementation is different. Additionaly, the bounds does not take into consideration bounding box attachments.