Make QGLWidget::drawTexture() respect painter's state if called outside a begin/endNativePainting() block

It would be nice if QGLWidget::drawTexture() worked without having to call begin/endNativePainting(), and respected the current painter's clip, opacity, render hints, and composition mode settings. Currently there's no way of drawing a GL texture within the QPainter framework.

The behaviour of drawTexture() when a QPainter is active needs to be clearly documented as well.