Fix QQuickWidget cleanup and invalidation sequence

Make sure the context/surface are still alive and current while destroying
the render control.

Task-number: QTBUG-39034
Change-Id: I6ff0069985a9121a63025bfb165493b3f003391d
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Destroy the FBO in QQuickWidget while a context is current

When creating the new FBO, it will try to restore the previous FBO
after initialization. The internal tracking of the current FBO goes
horribly wrong when a QOpenGLFramebufferObject is destroyed with
no context current. In 5.4 the problem is solved by removing the
tracking altogether. Here QQuickWidget is fixed to play nice and
destroy QOpenGLFramebufferObject always with the context current.

Task-number: QTBUG-39389
Change-Id: Ic6917696ed61f284d661a578c9c7f2e0673c412d
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Recognize "in" keyword in vertex shaders

Checking only for "attribute" is not sufficient: with newer GLSL versions
"in" is used.

Task-number: QTBUG-39143
Change-Id: Ibdb5940f5aba1485ecaad2e2c4a3aa7150af6d99
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>