Improve --image/-im flag

[mateosss]

See issues discussed on #57 with the current feature.

[heyzec]

For ease of reference, these are the current issues:

1. When script is rerun (either with new image or without any image at all), the old image will remain in /boot/grub/themes/Matter. A similar approach to what is being done for icons and fonts might be useful here.

2. Background image will not show when using with the -b -t flags for previewing.

[wiredmatt]

It worked perfectly for me, I even tested with multiple backgrounds, icons and fonts, what's the exact problem with this?

Tested on Manjaro Linux (EFI GPT filesystem):

[mateosss]

Indeed the feature works great. The issues were the two listed in the last comment from @heyzec, although they are background issues and do not affect normal usage.

For once, point 1 in that comment means you will have multiple unused images being stored in your /boot/grub/themes/Matter directory while point 2 forces you to test the theme by rebooting instead of the quicker testing procedure with -b -t flags.

[wiredmatt]

Interesting, the -b -t flags worked for me without issues. Manjaro uses a different version of grub2 (their own), so it might be the case that it just works with Manjaro, since I've tried previewing in Fedora and Arch, but couldn't preview the GRUB theme even without a background image.

Regarding point 1, you could make the script rename the background image to background.`jpg, so when it copies over to /boot/grub/themes/Matter, whatever there was before will be automatically replaced.