Crash at Qt3d teardown with ANGLE

My Qt3d applications started crashing at teardown after QTBUG-57496 introduced https://codereview.qt-project.org/179252 . The proposed https://codereview.qt-project.org/179636 did not fix the crash.

Steps to reproduce:

• Build tests\manual\scene3d-loader with VS2015 in debug.
• Run application over remote desktop or set QT_OPENGL=angle to trigger ANGLE back-end.
• Close window.
• Observe "Exception thrown: read access violation. d was 0xFFFFFFFFFFFFFF5F." crash in Qt3DCore::QAspectThread. Call stack and debugger screenshot after applying patch #179636 attached.

The crash is caused by the QOpenGLContext::d_ptr pointer having the value 0xdddddddddddddddd. This is an indication of use-after-free (http://stackoverflow.com/questions/370195/when-and-why-will-an-os-initialise-memory-to-0xcd-0xdd-etc-on-malloc-free-new).