Fullscreen does not work in combination with `position: "middle_center"

[ngnijland]

Steps to reproduce:

1. Use the following config:

{
            module: "MMM-text-clock",
            config: {
                fullscreen: true
            },
            position: "middle_center"
 },

2. Start MagicMirror

Expected result:
The clock module should be stretched from the top left to the bottom right corner.

Actual result:
Clock is not stretched and rendered in the center of the screen aligned to the top of the clock.