DepthTest flag on Graphics is set by Node but never cleared, misbehaving with multiple dirty regions

NGNode's render method calls g.setDepthTest(isDepthTest()). Because every node does it, one might be forgiven for never restoring the state. However, this *same* graphics object instance is reused from the rendering of one dirty region to the rendering of the next. If our policy is to manually restore graphics state after rendering, then we need to be sure to restore the depth test state as well, otherwise we will misbehave. If instead we don't guarantee that, then we need to use different graphics objects between dirty regions.