watchedLayers property
How many entries the layer watch's last signature had, or 0 if it never ran.
The trace the watch would otherwise not have, and without it a run that
never walked would report as a walk that costs nothing — which is the
shape Picture.toImageSync(targetFormat:) already cost this project once.
Non-zero says the composited subtree was described; its size says how much
of one.
Implementation
int watchedLayers = 0;