Add support for generating dummy DeviceIDs and WindowIDs (#738)

* Add support for generating dummy DeviceIDs and WindowIDs

* Fix linux

* Improve docs and move dummy to unsafe

* Strengthen guarantees a bit

* Add backticks to CHANGELOG.md

Co-Authored-By: Xaeroxe <xaeroxe@amethyst-engine.org>

src/platform/linux/mod.rs

@@ -66,12 +66,24 @@ pub enum WindowId {
     Wayland(wayland::WindowId),
 }
 
+impl WindowId {
+    pub unsafe fn dummy() -> Self {
+        WindowId::X(x11::WindowId::dummy())
+    }
+}
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum DeviceId {
     X(x11::DeviceId),
     Wayland(wayland::DeviceId),
 }
 
+impl DeviceId {
+    pub unsafe fn dummy() -> Self {
+        DeviceId::X(x11::DeviceId::dummy())
+    }
+}
+
 #[derive(Debug, Clone)]
 pub enum MonitorId {
     X(x11::MonitorId),

src/platform/linux/wayland/mod.rs

@@ -16,9 +16,21 @@ mod window;
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct DeviceId;
 
+impl DeviceId {
+    pub unsafe fn dummy() -> Self {
+        DeviceId
+    }
+}
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct WindowId(usize);
 
+impl WindowId {
+    pub unsafe fn dummy() -> Self {
+        WindowId(0)
+    }
+}
+
 #[inline]
 fn make_wid(s: &Proxy<wl_surface::WlSurface>) -> WindowId {
     WindowId(s.c_ptr() as usize)

src/platform/linux/x11/mod.rs

@@ -1262,9 +1262,21 @@ impl<'a> Deref for DeviceInfo<'a> {
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct WindowId(ffi::Window);
 
+impl WindowId {
+    pub unsafe fn dummy() -> Self {
+        WindowId(0)
+    }
+}
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct DeviceId(c_int);
 
+impl DeviceId {
+    pub unsafe fn dummy() -> Self {
+        DeviceId(0)
+    }
+}
+
 pub struct Window(Arc<UnownedWindow>);
 
 impl Deref for Window {