Added rendering mode parameter functionality

[ArthurDeveloper]

You can optionally change how the wallpaper will be rendered by giving a third argument, just like the example shown below:

$ ./loop_pngs.sh ~/Desktop/forest 0.05 f

In this case, it will stretch the gif if it ain't the exact proportions of the screen, for instance, if you used a 1366x768 gif in a 1980x1080 monitor it would have black borders in normal mode, however, with f the whole background will be filled (although some of the image quality might be lost)

[sdaveas]

Thank you for the contribution and sorry for the super late response!

Could you please remove the generated *.png files from the git history?

We generally don't need to track all gifs (the ones included are just some defaults). But if you do want to track this 🦍, adding the original .gif file in /gifs should be fine (as the individual .png files can be created with generate_pngs.sh).