Default to threaded when using opengl32

This appplies only when using opengl32.dll. When using ANGLE or
when using another non-opengl32 DLL, the "windows" single-threaded
loop is chosen, just like before.

One badly written autotest is fixed. We must never assume that
performing some QQuickWindow related work will leave a context
current on the main thread. The assumption worked by accident with
the single-threaded render loop.

[ChangeLog][QtQuick] Changed to use the threaded render loop by
default on Windows when running with desktop OpenGL (opengl32.dll).

Change-Id: I21798622c19ca510a97a96c19c1e70e29f086e3a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>