Hey, just have some quick questions :)

Are you still working on this? (haven't seen any updates or heard from you of late)

Why does it seem like tangent is not supported in the GLSL ES version? I found a way to check the scripts in the extension in order to find the actual vertex format. I originally thought I was just setting  the input up wrong in the shader but I found that the ES support was commented out in the extension for some reason. I wouldn't dare to try and change anything in your extension cuz I'm pretty sure I'd break something that way and also I don't want to mess with other people's code in general xD

That's all really ^^

Hope to hear from you :)

Only a year late (oof, sorry about that), but it looks like I am going to be focusing on this on my weekends shortly, since something important has broken when used with newer runtimes. 

I would guess the tangent stuff was commented out due to the weird way gamemaker handles format types and GLSL ES, but it's been so long I don't remember the specifics. Hopefully the rewrite will address this