core: Wayland: Utilize Client Side Decorations when Server Side Decorations are not supported

[RonaldZielaznicki]

As discussed in #1275, not all window managers support Server Side Decorations.

Work to be done

• - As a stopgap, raise and error when SSD is not supported. (Will cause Mach to fallback to X11)
  • Wayland should error when no server side decoration #1292
• - Add LibDecor as a dependency
  • Add libdecor #1294
• - Integrate LibDecor into src/core/linux/Wayland.zig

On Integrating LibDecor

As discussed here: LibDecor should be the default.

• If LibDecor is not found, we use SSD.
• If we can't use SSD, we throw an error

There are two cases where SSD won't be available:

1. Compositor has not implemented zxdg_decoration_manager_v1_get_toplevel_decoration

   • Mutter, Weston, Gamescope, etc. etc.

2. or when zxdg_decoration_manager_v1_get_toplevel_decoration is implemented, but the compositor signals CSD.

   • Even if we tell the compositor we prefer SSD, it can still use CSD.
   • Cosmic (at time of writing) is the only compositor I know of that does this. Even if they do add SSD support as they work through their Alpha, we should still support this case because it's what the protocol expects.
   • It'll tell us with a configure event. We should respect the mode specified during the event.