I was a little surprised to not find any top-down portal anything, just side-scrolling. Functionally the same i suppose. Anyhow, I took a stab at it and came up with a working shader, and a view transition system that mostly works.
Thought I'd share it before I possibly mangle it.
https://github.com/bobbysoon/topdown_portalsDraw lines with the mouse to place the portals. Move with WASD. Pan view with the middle mouse button. Scroll wheel to zoom.