A view’s bounding box, described in the view’s own coordinate system.
Concretely, |AlignedExtent| describes the minimal and maximal points of a
view’s bounding box, which is rectangular and axis-aligned.
A view bounding box, described in another view’s coordinate system.
Concretely, |RotatableExtent| describes the origin, size, and rotation angle
about the origin, for a view’s bounding box.
The maximum number of ViewTreeSnapshots a client can expect in a Watch call’s
response. The number of ViewTreeSnapshots can be less than BUFFER_SIZE when
the size of a Watch call’s response exceeds the limit of a FIDL channel.