QRawFont: improve the thread-safety checks

384388f2 introduced some checks, and used an assignment in an assert;
that sets off compiler warnings about expressions with side effects
into an assertion. Hence, that code needs to be reworked a bit.

Unfortunately, there's no single define we can use to know if
assertions are enabled or not in Qt, so simply use QT_NO_DEBUG
to enable/disable those checks. The actual "thread" data member
is kept around to avoid break ABI depending on debugging flags.

Change-Id: I8b07e7ff6f81359d6b0653a1d9cc2b720541d1b9
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>
Remove a memory leak in the custom sort filter model example

Change-Id: Ie14f07bc2ee2215da2a81a75d816d4763fac085f
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>