GL_ATI_texture_env_combine3
http://www.opengl.org/registry/specs/ATI/texture_env_combine3.txt
GL_ATI_texture_env_combine3
	GL_MODULATE_ADD_ATI 0x8744
	GL_MODULATE_SIGNED_ADD_ATI 0x8745
	GL_MODULATE_SUBTRACT_ATI 0x8746
