Optimise render_y function

[ooyendyk]

Currently every second the screen has to clear and re-render every character on screen. At high resolutions and on slower devices, this results in slow performance, screen flickering, and epic time drift.

A much more efficient way to do things would be to simply keep the old characters, and add new ones as needed. This would take the run-time down from O(n^2) (ish) to sub O((n^2)+1).

It might be a little tricky dealing with old render_time string though.