feat: add renderable event to CoreNode

[wouterlucas]

Added a new event to a CoreNode to track when a node becomes renderable or not.

This will fire an event if:

• The node dimensions change
• Opacity change
• Texture is added/removed
• Bounds status change

And the renderer decides to take it out or put it into the active render tree.
This is useful when trying to detect if a node is being actively rendered or not, for lifecycle management.