Cropped output on large monitor

### Issue
On a large 4k monitor, if I open a program that takes the whole screen like Vim, Mutt, or Tmux, the bottom of the screen stays black (my background colour).

It doesn't seem to be related to #824 as key presses will not do a complete redraw and the number of lines displayed is consistent.

The interesting thing is, if I reduce the window vertically (i.e. reduce the number of lines) the same number of lines will be displayed, but if I reduce the window horizontally, more and more lines are displayed. If I increase the font size, more lines are displayed.

I did a test by filling the screen with `y` and it seems that the maximum number of `y`s displayed is 65536 (2^16).

So it makes me think that it went past the maximum number of characters (cells? glyphs?) to be displayed. I tried to look in the code to see if there was a type that wasn't big enough but to no avail. If you have any hint, I'm happy to invest more time.

Or maybe it is intended to be like this? It's too bad because I like to have Tmux in fullscreen, maybe a warning in the README or in the program would be nice.

Any way, thanks a lot for Alacritty! ❤️

### System

OS: macOS
Version: `alacritty 0.4.2 (f68de37)` and `alacritty 0.5.0-dev (81ce935)`

### Logs

Font/Terminal size:
```
Device pixel ratio: 1
Cell Size: 5 x 11
Padding: 0 x 0
New num_cols is 768 and num_lines is 196
Width: 3840, Height: 2160
Font size changed to Size(18) with DPR of 1
```