by frag.machine » Wed Jan 24, 2018 11:01 am
Disclaimer: I don't know glsl. That said, I would start looking for a way to DISABLE any z-buffer test/writing. Reading from the z-buffer probably is a very expensive operation in terms of performance, if it's even supported in glsl.
I know FrikaC made a cgi-bin version of the quakec interpreter once and wrote part of his website in QuakeC 
(LordHavoc)