Unify defaultFramebufferObject between QOpenGLContext and QOpenGLWidget and QOpenGLWindow

The fact that QOpenGLWidget provides its own defaultFramebufferObject() function is somewhat confusing. Most people try to use QOpenGLContext::defaultFramebufferObject() instead which of course knows nothing about QOpenGLWidget and QOpenGLWindow and such.

See http://qt-project.org/forums/viewthread/50427/ for example

Additionally, this breaks functions like

• QOpenGLFramebufferObject::release()
• QOpenGLFramebufferObject::bindDefault()
• QOpenGLFunctions::glBindFramebuffer with fbo == 0

that rebind the context's defaultFramebufferObject() but that is not the correct one (it is always 0 except on iOS) when performing rendering into a QOpenGLWidget.