Fix max scroll width in react editor

This should fix #2666 

It basically calculates the scrollWidth by measuring the longest line. 

It feels dirty, though, because I have to change a bunch of tests to first call `DisplayBuffer.updateAllScreenRows`. Why is this? It seems `updateAllScreenRows` is called in the DisplayBuffer's constructor. I'm looking more into this. How can I make this not suck @nathansobo?

It fixes #2666 except in one case: initial load. I assume this is because lines are measured before styling has been applied. Looking into this as well...