GL_EXT_texture_compression_rgtc
http://developer.download.nvidia.com/opengl/specs/GL_EXT_texture_compression_rgtc.txt
	GL_COMPRESSED_RED_RGTC1_EXT 0x8DBB
	GL_COMPRESSED_SIGNED_RED_RGTC1_EXT 0x8DBC
	GL_COMPRESSED_RED_GREEN_RGTC2_EXT 0x8DBD
	GL_COMPRESSED_SIGNED_RED_GREEN_RGTC2_EXT 0x8DBE