effect: Rework the OffscreenEffect painting

effect: Rework the OffscreenEffect painting

The OffscreenEffect should set up the off screen draw buffer so that it
has the same projection and modelview as if it where on screen; we
achieve that by setting up the viewport to be the same size of the stage
but with an initial offset given by the left-most vertex of the actor.

When we paint the texture attached to the FBO we then set up the
modelview matrix of the on screen draw buffer so that it's the same as
the stage one: this way, the texture will be painted in screen
coordinates and it will occupy the same area as the actor would have
had.