Add support for Vulkan on Wayland

The variable VK_USE_PLATFORM_WAYLAND_KHR is actually used by the Vulkan
header, so use it here too.

Utilities/variant_visitor.hpp

@@ -0,0 +1,45 @@
+#ifndef MAPBOX_UTIL_VARIANT_VISITOR_HPP
+#define MAPBOX_UTIL_VARIANT_VISITOR_HPP
+
+#include <utility>
+
+//namespace mapbox {
+//namespace util {
+namespace std {
+
+template <typename... Fns>
+struct visitor;
+
+template <typename Fn>
+struct visitor<Fn> : Fn
+{
+    using Fn::operator();
+
+    template<typename T>
+    visitor(T&& fn) : Fn(std::forward<T>(fn)) {}
+};
+
+template <typename Fn, typename... Fns>
+struct visitor<Fn, Fns...> : Fn, visitor<Fns...>
+{
+    using Fn::operator();
+    using visitor<Fns...>::operator();
+
+    template<typename T, typename... Ts>
+    visitor(T&& fn, Ts&&... fns)
+        : Fn(std::forward<T>(fn))
+        , visitor<Fns...>(std::forward<Ts>(fns)...) {}
+};
+
+template <typename... Fns>
+visitor<typename std::decay<Fns>::type...> make_visitor(Fns&&... fns)
+{
+    return visitor<typename std::decay<Fns>::type...>
+        (std::forward<Fns>(fns)...);
+}
+
+}
+//} // namespace util
+//} // namespace mapbox
+
+#endif // MAPBOX_UTIL_VARIANT_VISITOR_HPP