Rather than when the data is received. Source compatibility is
achieved through double-decompressing the data. This lets us know
how many bytes are available just as before but without having the
uncompressed data left in memory.
Fixes: QTBUG-83269
Change-Id: I352bd09581614c582e4628243e2a0e895ba4946b
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
