Portal refactor & Anchor implementation

[creativecreatorormaybenot]

Description

• Implements passing the root bounds to portal anchors, i.e. passing the portalRect (renamed from overlayRect) to getFollowerOffset (renamed from getSourceOffset).
• Implements handling of off-screen cut-off with a dedicated example.
• Refactors the library to clean up all concepts to use only Portal, PortalTarget, PortalFollower, and Anchor.
• Updates all documentation and examples.

Implementation details

The passing of root bounds is achieved using a custom FollowerLayer implementation that takes a callback for computing the linkedOffset. This allows making the linkedOffset depend on the offset of the leader layer (via CompositedTransformTarget). And this in turn allows to make the linkedOffset depend on the theaterRect, which requires only the relative top-left offset to the targetRect and the size of the theater, which we can obtain via the _RenderPortalTheater render box.

Tests

This PR only contains two unit test cases, but I did not feel the need to add more test cases since these are already the cases that cover the "most possible" edge cases, i.e. a portal that is not the root of the app and portal entry sizes that differ from the portal and child sizes.

Fragility

Doing this with TDD really helped me to come to a non-fragile implementation. Initially, I thought it would be enough to call localToGlobal with Offset.zero and the theater render box ancestor. However, this does not work because it does not respect the offset of the LayerLink. After that, I thought that I could access the link.leader in paint directly. Using a non-null assertion worked fine in my example app case, however, the unit tests revealed that this was actually not safe to do on the first paint call. This is because the layer link leader only attaches itself after the painting phase. And this is how I finally arrived at creating a custom FollowerLayer, which allows to access the leader layer offset in the composition phase.

This is why I believe that the new implementation is non-fragile and solid.

We can of course add more test cases - it cannot hurt :)

[creativecreatorormaybenot]

Current state: the new concepts are introduced and applied everywhere. I have touched all of the docs and examples to use the new concepts and it all seems coherent to me.

What is still open is handling the off-screen cut-off.

[creativecreatorormaybenot]

Closing this here as the changes have been merged upstream in fzyzcjy#44.