On Unix, add option to pick backends

Add features 'x11'  and 'wayland' to pick backends on Linux/BSD, with
both enabled by default.

Fixes #774.

src/platform_impl/linux/x11/mod.rs

@@ -426,6 +426,7 @@ impl<T: 'static> EventLoop<T> {
 pub(crate) fn get_xtarget<T>(target: &RootELW<T>) -> &EventLoopWindowTarget<T> {
     match target.p {
         super::EventLoopWindowTarget::X(ref target) => target,
+        #[cfg(feature = "wayland")]
         _ => unreachable!(),
     }
 }
@@ -493,9 +494,21 @@ 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 {

src/platform_impl/linux/x11/window.rs

@@ -653,6 +653,7 @@ impl UnownedWindow {
                     Fullscreen::Borderless(RootMonitorHandle {
                         inner: PlatformMonitorHandle::X(ref monitor),
                     }) => (None, monitor),
+                    #[cfg(feature = "wayland")]
                     _ => unreachable!(),
                 };