https://bugs.freedesktop.org/show_bug.cgi?id=57875
--- Comment #28 from Marek Olšák maraeo@gmail.com --- Stefan, is the extension for implementing the POSITIONT shader semantic? Would a gl_Position output modifier also work for you?
For example:
#extension GL_MESA_xxx : require
pretransformed gl_Position;
void main() { ... etc.