There’s currently no way to do remote debugging for vertex or fragment programs, though it could be done at least in software now.
Why not introduce some debugging extension?
One could use it like this:
then the various states at each stage of the curent fragment program would be saved for point at screen coordinates (.2,.3) in the current context, and would be queriable with something like this:
…so one could trace in the whole program for a resqueted point.
Something similar could be done for vertex program (or maybe generalized).