A copy of important spatial node data to use during hit testing. This a copy of
data from the SpatialTree that will persist as a new frame is under construction,
allowing hit tests consistent with the currently rendered frame.
Defines the immutable part of a hit tester for a given scene.
The hit tester is recreated each time a frame is built, since
it relies on the current values of the spatial tree.
However, the clip chain and item definitions don’t change,
so they are created once per scene, and shared between
hit tester instances via Arc.