[windowthumbnail] Verify that the relevant GLContext exists, not any

glXGetCurrentContext checks if any GLContext exists and is current.

resolveGLXFunctions uses window->openglContext()

One can hit a situation that you are using software rendering for the
window, yet someone else has created a GL Context. This then crashes.

src/declarativeimports/core/windowthumbnail.cpp

@@ -376,7 +376,7 @@ void WindowThumbnail::iconToTexture(WindowTextureNode *textureNode)
 #if HAVE_GLX
 bool WindowThumbnail::windowToTextureGLX(WindowTextureNode *textureNode)
 {
-    if (glXGetCurrentContext()) {
+    if (window()->openglContext()) {
         if (!m_openGLFunctionsResolved) {
             resolveGLXFunctions();
         }