GL_ATIX_texture_env_combine3
http://www.ati.com/developer/atiopengl.pdf
GL_ATIX_texture_env_combine3
	GL_MODULATE_ADD_ATIX 0x8744
	GL_MODULATE_SIGNED_ADD_ATIX 0x8745
	GL_MODULATE_SUBTRACT_ATIX 0x8746