Windows QPA: introduce a way to disable redirection surface

Why do we need this feature: to give user the possibility to explore
different interesting graphics features (currently only compatible
with D3D with the new FLIP model).

Why do we need to modify QPA directly: touch this flag after the
window has been created has no effect. It can only be used when we
call CreateWindow(), so we can't add this flag through SetWindowLong().
However, Qt doesn't provide any public or private interface to
control how Qt call CreateWindow(), and providing such interface
is also dangerous. Using environment variable to control this behavior
is the simplest solution and is commonly used inside Qt code base.

Change-Id: I12440ed498d97cc17640e6c7498e42770b813737
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -834,6 +834,10 @@ void WindowCreationData::fromWindow(const QWindow *w, const Qt::WindowFlags flag
         // NOTE: WS_EX_TRANSPARENT flag can make mouse inputs fall through a layered window
         if (flagsIn & Qt::WindowTransparentForInput)
             exStyle |= WS_EX_LAYERED | WS_EX_TRANSPARENT;
+
+        // Currently only compatible with D3D surfaces, use it with care.
+        if (qEnvironmentVariableIntValue("QT_QPA_DISABLE_REDIRECTION_SURFACE"))
+            exStyle |= WS_EX_NOREDIRECTIONBITMAP;
     }
 }