Added monitor span support

[BizarreMachinist]

Hey there!

I've added the ability to have the video stretch across dual monitors (an probably more, but I can't test it).
This is for when you make a video that is the combined width of both monitors. Previously it would squish the width and display a copy on each monitor.
This can be done by adding an optional --span or -s after the file name.
I've also made it throw an error if no arguments are given. This doesn't stop you from adding more than the file name and -s, but it'll simply ignore everything after.

I tried to keep your formatting style while editing. Let me know what you think. This is kinda my first pull request :D

Have a glorious day!

[Ninlives]

Hi @BizarreMachinist , sorry for the very late response and thanks for your help, I haven't look into this project for a long time and I decided to update it a bit during this holiday. However I cannot test your pr locally since I do not work with dual monitors, I hope someone else can confirm this pr works :)

[Truemmerer]

Unfortunately, this does not work for me. I only get a black screen.

GNOME 42
Fedora 36
X11
2 Monitors