GL_S3_s3tc
http://www.opengl.org/registry/specs/S3/s3tc.txt
GL_RGB_S3TC 0x83A0
GL_RGB4_S3TC 0x83A1
GL_RGBA_S3TC 0x83A2
GL_RGBA4_S3TC 0x83A3
GL_RGBA_DXT5_S3TC 0x83A4
GL_RGBA4_DXT5_S3TC 0x83A5