GL_ARB_texture_compression_rgtc
http://www.opengl.org/registry/specs/gl/ARB/texture_compression_rgtc.txt
GL_ARB_texture_compression_rgtc
	GL_COMPRESSED_RED_RGTC1 0x8DBB
	GL_COMPRESSED_SIGNED_RED_RGTC1 0x8DBC
	GL_COMPRESSED_RG_RGTC2 0x8DBD
	GL_COMPRESSED_SIGNED_RG_RGTC2 0x8DBE