mach: add headless option (#450)

Co-authored-by: Stephen Gutekanst <stephen.gutekanst@gmail.com>

src/structs.zig

@@ -34,6 +34,10 @@ pub const Options = struct {
     /// Fullscreen window.
     fullscreen: bool = false,
 
+    /// Headless mode. Once set, this cannot be unset (because e.g. Wayland does not support showing
+    /// a window after it was hidden.)
+    headless: bool = false,
+
     /// Monitor synchronization modes.
     vsync: enums.VSyncMode = .double,