change resolution before shadec_init
This doesn`t work for me because I use different resolutions for the main menu and the game. Plus you can toggle the resolution in the ingame menu. All this works by video_set.
Then, you probably have to init shade-c on each level start.
EDIT: Ups, you've been faster!
You can use it with c-script but it is a bit tricky and I always forget how to do it :P
Maybe, you could write it down, when you remember it! Just in case!
Hope you don't mind my commentary. I know you are doing already sooo much!