This is an alternative to DRAWBUFFERS needed to write to colortex10 and above. Care must be taken to ensure that RENDERTARGETS and DRAWBUFFERS interact properly, ie that this type of file would be parsed the same as OptiFine:

void main() {
    /* DRAWBUFFERS: 013 */
    /* RENDERTARGETS: 0, 1, 2 */
}

In this example, the RENDERTARGETS directive comes later, and takes precedence, since DRAWBUFFERS and RENDERTARGETS are two different ways of setting the same values.

This was just implemented by Justsnoopy30: 61f5ac8