QNX: Fix flushing the backing store to non-owner windows

QNX: Fix flushing the backing store to non-owner windows

Multiple windows can share the same backing store, which we did not
take into account. Don't ignore the window parameter in flush() to fix
that.

Change-Id: I4f98bba34d4da9134163e478cb78cab4ca0358d2
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>