winrt: Skip context validation in ANGLE

The flag causes context validation to be skipped by ANGLE. If validation
is active nothing is rendered on Windows Phone (just a black screen) for
widget applications.
Everything works as expected without validation so we do without it.

Change-Id: I6f9ea249b653ba5a602bc33e75105c57b686b92d
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>

src/plugins/platforms/winrt/qwinrteglcontext.cpp

@@ -138,6 +138,7 @@ void QWinRTEGLContext::initialize()
         EGL_CONTEXT_CLIENT_VERSION, d->format.majorVersion(),
         EGL_CONTEXT_MINOR_VERSION_KHR, d->format.minorVersion(),
         EGL_CONTEXT_FLAGS_KHR, flags,
+        EGL_CONTEXT_OPENGL_NO_ERROR_KHR, true,
         EGL_NONE
     };
     d->eglContext = eglCreateContext(g->eglDisplay, d->eglConfig, d->eglShareContext, attributes);