[wayland] Enforce basic render loop

Looks like Qt 5.5 changed the default back to threaded and that can
result in dead locks of the compositor. So better enforce to basic
for the time being.

Another reason to have an own QPA to get rid of these "regressions".

main_wayland.cpp

@@ -461,6 +461,7 @@ int main(int argc, char * argv[])
     qunsetenv("QT_IM_MODULE");
     qputenv("WAYLAND_SOCKET", QByteArray::number(server->createQtConnection()));
     qputenv("QT_WAYLAND_DISABLE_WINDOWDECORATION", "1");
+    qputenv("QSG_RENDER_LOOP", "basic");
     KWin::ApplicationWayland a(argc, argv);
     a.setupTranslator();
     a.setProcessStartupEnvironment(environment);