[Design-team] GRUB background preview overlay

[Marc Stewart]

GRUB background screen creators,

To save time when testing out new designs, I've added this overlay file
to the wiki:
https://fedoraproject.org/wiki/File:Grub_background_preview_overlay.png

You can use it check for positioning, legibility, etc, without having to
reboot (or start a VM) each time. Just add it as a new layer for an
instant preview.

Marc