Center the grid inside the window

Currently alacritty always puts the grid at the top-left position of the
window. The only distance to the top-left window border is set by the
padding in the config.

However the grid always has a fixed size, and if a cell doesn't
completely fit the screen anymore, the padding at the bottom right
window corner can be significantly bigger than the padding at the top
left.

To fix this whenever there is more space left and there would usually be
a bigger padding at the bottom right, the space is now split up and
added to the padding.

This should always center the grid inside the window and make sure all
borders have the same padding from the text area.

This screenshot shows how it has been until now:
![Before](https://u.teknik.io/kRJwg.png)

Here is how it looks now:
![After](https://u.teknik.io/m4puV.png)

This fixes #1065.

0 files changed, 0 insertions, 0 deletions