[Improvement]: Support small-sized logo splashes and adaptive icon background images

[valknarthing]

Hello,

first of all thank you for providing this generator, i don't think there is another comparable tool out there!

I have two issues, which i think could be improved:

1. Support logo splashes: The problem is currently i have to specify a 3000x3000 image with set-splash --resize=center with a small logo in the center and huge outer area of transparent pixels to achieve the wanted result of displaying just a small logo in the center (or i did something wrong).

2. Support adaptive icon background images: It would be nice to provide a background image to the set-icon generator to generate more sophisticated adaptive icons (e.g. gradient background / foreground with shadow)

Thanks! Sebastian

[Kilowhisky]

+1 for background images for adaptive icons