feat: allow customising the color of the fullscreen backdrop

[Rolv-Apneseth]

Small PR to allow customising the colour of the fullscreen backdrop, instead of just having a hard-coded black colour. Figured this could be a potential solution for people (like me) who want transparency for fullscreen windows (see #1399), while also giving a bit more customisation to others too. Unfortunately, using a transparent colour lets you see that layer shells like waybar don't get hidden, but I believe handling that should be a separate PR.

Totally understand if you don't want this as a feature for niri, no hard feelings. If you are open to it though, just let me know if you want any changes made.

Edit: Forgot to briefly describe how this is actually achieved:

Added a fullscreen-backdrop-color option under layout, which can be set to any colour (including a transparent one):

layout {
    fullscreen-backdrop-color "transparent"
}

The backdrop colour also updates when the config is saved, as would be expected.

[saygo-png]

Tested the patch and it works well. Thanks!