GL_NV_texture_env_combine4
http://oss.sgi.com/projects/ogl-sample/registry/NV/texture_env_combine4.txt
	GL_COMBINE4_NV 0x8503
	GL_SOURCE3_RGB_NV 0x8583
	GL_SOURCE3_ALPHA_NV 0x858B
	GL_OPERAND3_RGB_NV 0x8593
	GL_OPERAND3_ALPHA_NV 0x859B
