Fix in qt backend for GlWidget shining through when it just becomes visible.
Fix in qt backend for GlWidget shining through when it just becomes visible.

pbfy0/visvis,chrisidefix/visvis,pbfy0/visvis,chrisidefix/visvis,Alwnikrotikz/visvis,pbfy0/visvis,chrisidefix/visvis,Alwnikrotikz/visvis,Alwnikrotikz/visvis