readBacks property
How many times the host has read the backdrop back for glass that reads
it (GlassHost.adaptive) — zero for ever on a host without it, which is
the observable that says the feature costs nothing when off.
Counted when a read is started rather than when it lands, because the start is what was paid for: a read-back still in flight when the host goes away cost its copy all the same.
Implementation
int readBacks = 0;