The big problem with using GLSL for filters and fill layers is that it is easy to sent things to the GPU, but it’s not easy to get it back from the GPU to the RAM without serious slow down, to the point we are not sure it wouldn’t end up being super slow instead.
On top of that, the question remains how long openGL is going to stay around, on Windows we are constantly dealing with driver bugs, and Apple hates openGL. Right now our openGL integration is rather superficial (we only sent things to the GPU), introducing GLSL for fill and filters will force more integration.
Finally, to pull this back to seexpr, this was a library already written by Walt Disney Animation studios, Amyspark just integrated it. Scheme would have definitely required them to write a whole API from scratch which meant we wouldn’t have had all the other fill layer improvements we got (Multithreading, better preview).