Improve hit-test of thin widgets, and widgets across layers

[emilk]

When there are multiple layers (e.g. with custom transforms) close to each other, the hit test code used to only consider widgets in the layer directly under the mouse. This can make it difficult to hit thin widgets just on the outside of a transform layer.

This PR fixes that.

It also prioritizes thin widgets, so that if there is both a thin widget and a thick widget under the mouse cursor, you will always hit the thin widgets, even if the thin widgets is layered behind the thick one. This makes it easier to hit thin resize-handles.

In theory this should allow us to make resize_grab_radius_side and resize_grab_radius_corner smaller in a future PR, if we want to.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5468-emilkimprove-cross-layer-hit-test
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.