[Feature Request] Pseudo wide-screen core support

Implement the (optional) capability for cores to display in pseudo wide-screen.

I'm assuming that a core would need to be updated to specifically support this capability - but that libretro would abstract as much as possible. 

Description and source code can be found at https://kzone.winosx.com/posts/widegb-playing-game-boy-games-on-wide-screens/