This is tricky because you have a lot of things going on all at once and a lot of older code mixed with a lot of newer code. ftransform() might not even exist in version 4.5 for example. I think it would probably be a very good idea if you followed a modern tutorial that just draws a single triangle to the screen. Once you have that you can make it a quad and once you have that you can put your Julia shader back in.
There's a bunch of tutorials here:
http://ogldev.atspace.co.uk/index.html
Take a look at tutorials 3, 4 and 5.
Is there a work-around for pass thru a gl_position to some other variable without the gl_vertex?
The tutorials above will show you how to do this.