Expose QPlatformWindow::invalidateSurface as a virtual function.

This can be quite useful on some embedded systems to free up
graphics memory when windows are not used. QEglFSWindow already
implements the function.

Change-Id: I79b08efbd3c67d7be34df6a0e12dd184a92d48c5
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
QOpenGLWidget and new-style compositing on eglfs

Integrate with QOpenGLTextureBlitter, QOpenGLWidget and friends.

Change-Id: Ic2867b713a21a3d2820d546174fc9164b3dd220c
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>