Add a new queriable resource limit value MaxAsyncReadbackFrames. Change
the autotest to rely on this instead of relying on the unspecified,
works-by-accident relation between readbacks and FramesInFlight. This
way even if the behavior diverges in some backend in the future, clients
(well written ones, that is), will continue to function correctly.
Also clarify the docs for FramesInFlight, and change d3d and gl to return
the correct value (which is 1 from QRhi perspective; the expanded docs now
explain a bit what this really means and what it does not).
Change-Id: I0486715570a9e6fc5d3dc431694d1712875dfe01
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
efba2530f9