Keep the internal state about the bound fbo up to date

When binding an FBO directly via glBindFramebuffer, the QOpenGLContext's
internal current_fbo, that is maintained by QOpenGLFramebufferObject,
becomes out of sync. This will lead to QOpenGLFramebufferObjects thinking
they are still bound.

This fix prevents ShaderEffect items in QQuickWidget from complaining
about "'recursive' must be set to true when rendering recursively". Ideally
the entire tracking of the current fbo should be removed, since it cannot
be done robustly enough, but that will likely happen in 5.4 only.

Task-number: QTBUG-38994
Change-Id: I39015d90bb2e0985d2a084b79ffe2004a9bc9f07
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>