GL_EXT_texture_env_combine
http://oss.sgi.com/projects/ogl-sample/registry/EXT/texture_env_combine.txt
	GL_COMBINE_EXT 0x8570
	GL_COMBINE_RGB_EXT 0x8571
	GL_COMBINE_ALPHA_EXT 0x8572
	GL_RGB_SCALE_EXT 0x8573
	GL_ADD_SIGNED_EXT 0x8574
	GL_INTERPOLATE_EXT 0x8575
	GL_CONSTANT_EXT 0x8576
	GL_PRIMARY_COLOR_EXT 0x8577
	GL_PREVIOUS_EXT 0x8578
	GL_SOURCE0_RGB_EXT 0x8580
	GL_SOURCE1_RGB_EXT 0x8581
	GL_SOURCE2_RGB_EXT 0x8582
	GL_SOURCE0_ALPHA_EXT 0x8588
	GL_SOURCE1_ALPHA_EXT 0x8589
	GL_SOURCE2_ALPHA_EXT 0x858A
	GL_OPERAND0_RGB_EXT 0x8590
	GL_OPERAND1_RGB_EXT 0x8591
	GL_OPERAND2_RGB_EXT 0x8592
	GL_OPERAND0_ALPHA_EXT 0x8598
	GL_OPERAND1_ALPHA_EXT 0x8599
	GL_OPERAND2_ALPHA_EXT 0x859A