Unbind the FBO.

def unbind(self):
        """Unbind the FBO."""
        # Unbind the FBO
        if self.texture.mipmap:
            with self.texture:
                self.texture.generate_mipmap()

        gl.glBindFramebufferEXT(gl.GL_FRAMEBUFFER_EXT, 0)

        # Restore the old viewport size
        gl.glViewport(*self._old_viewport)