Make screen corners appear in the right place on multi-screen systems

[lunaneff]

Fixes #2, partially #4

I also tried fixing #3, but it looks like panel_corners.js is currently completely broken, as when I commented out the rendering code in screen_corner.js I didn't see any rounded corners (even in the overview). I wasn't really able to understand how it's supposed to work, so I can't fix that

edit: actually, it looks like panel corners do work, but only on Wayland (and I didn't notice because I used Xorg so I can reload without relogging)

edit 2: actually no, I accidentally rebooted into an ostree deployment which is still on gnome-shell 41

[aunetx]

Thank you very much, I don't have an external monitor to test it; however it looks like exactly the right thing to do here.

What do you mean by fixing partially #4? Is there still any issue after this PR?

And for #3, I will see what I can do; however I don't fully understand the issue. Thanks you very much!

[lunaneff]

To be honest, when I created #4, I didn't properly look at #2 and #3, so I basically made a single issue about both.

I'm pretty sure the issue with #3 is that there are no corners below the panel, unlike gnome-shell pre-42:

[aunetx]

Uh that's not expected at all... Does it work when you use only one monitor?

[lunaneff]

Nope, a single monitor doesn't work either

Distro is Fedora Silverblue Rawhide, build from 2022-02-20, gnome-shell 42.beta