1. unfilled clipped triangle is not ok (edgeflag?!?)
2. glCullFace(GL_FRONT_AND_BACK) is not 100% correct
3. secondarycolor:
    fragment -> alphatest -> stenciltest -> depthtest -> blending
    OpenGL:
	primitive -> texturing -> colorsum -> fog -> fragments (blending)
    SAGE:
	primitive -> texturing -> fog -> fragments (blending) -> colorsum
