Overhaul Scrolling algorithm

[luposmi]

Currently, the scrolling has a lazy approach where the browser only scrolls if the cursor inside nvim is outside that what's shown on the screen. However, i find this quite anoying. For example, if i am at the top of the document and jump down to a section I want to read, only the headline is shown.

I would personally much prefer it if the "lazy" section in the browser is not the actual screen height, but, for example 30% and if the cursor gets outside that section, either centralize or move down as it's currently the case. Centralizing would be less smooth, but more responsive. This would actually be the same way vim handles it internally. The other option would be to just keep the cursor always in the middle.

[jannis-baum]

Hello! Did you see jannis-baum/Vivify#146? As far as I can tell, this issue is a duplicate of that. If not, feel free to specify things in more detail and reopen this :)