mint-arena: Fix mouse position when open/close console in-game

Mouse position was moving right each time I opened/closed the console.
Only happened in 4:3. I think I remember similar issue, I think fixing it
in wide screen broke 4:3. Now it works in both 4:3 and 16:9.

I move the converting to/from window coords to cg_main.c instead of
duplicating in both UIs.