Change the renderable constructor signature to include z ordering information

[obiot]

Following #664, were we updated the renderable component to use a 3d Vector as position vector, the constructor signature should also be changed to more easily specify the desired value.

[obiot]

I propose to keep this one for the next major release (4.0.0), I believe we already have enough breaking changes for this version :)

[obiot]

Closing this as effectively already addressed.

The Renderable class already uses an ObservableVector3d for pos, so pos.z is available on all renderables. Additionally:

• depth property (getter/setter) is available on all renderables as an alias for pos.z
• settings.z is supported by Sprite, Text, BitmapText, and other subclasses — allowing z to be specified at construction time via the settings object
• Container.addChild(child, z) accepts an optional z parameter for setting depth on insertion
• Container.autoDepth automatically assigns z values to children

These mechanisms cover all common use cases for specifying z-ordering information, both at construction time and dynamically. Changing the base Renderable(x, y, width, height) constructor signature to add a 5th positional parameter would touch 22+ subclasses for minimal practical benefit over the existing settings.z approach.