FR: Modular Post_processing.tbl with modular shader support

Currently, the post_processing.tbl is non-modular. If it were to be made modular, there needs to be a similar feature to allow modularization of the underlying post-f/v.sdr that supply the shaders for the post processing effects. If done well, these modular shaders would also allow to decouple custom effects in a way that they don't break if the in-engine post processing shader changes. This would still not give any guarantees for forwards-compatibility of the shaders though, since we might eventually switch rendering pipelines.