Vulkan backend (#2)

This commit adds the Vulkan backend and SDL2 integration, and provides the `vulkan` example project.

src/DearImGui/SDL/Vulkan.hs

@@ -0,0 +1,45 @@
+{-# LANGUAGE BlockArguments #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE QuasiQuotes #-}
+{-# LANGUAGE TemplateHaskell #-}
+
+{-|
+Module: DearImGui.SDL.Vulkan
+
+Initialising the Vulkan backend for Dear ImGui using SDL2.
+-}
+
+module DearImGui.SDL.Vulkan
+  ( sdl2InitForVulkan )
+  where
+
+-- inline-c
+import qualified Language.C.Inline as C
+
+-- inline-c-cpp
+import qualified Language.C.Inline.Cpp as Cpp
+
+-- sdl2
+import SDL.Internal.Types
+  ( Window(..) )
+
+-- transformers
+import Control.Monad.IO.Class ( MonadIO, liftIO )
+
+
+C.context Cpp.cppCtx
+C.include "imgui.h"
+C.include "backends/imgui_impl_vulkan.h"
+C.include "backends/imgui_impl_sdl.h"
+C.include "SDL.h"
+C.include "SDL_vulkan.h"
+Cpp.using "namespace ImGui"
+
+
+-- | Wraps @ImGui_ImplSDL2_InitForVulkan@.
+sdl2InitForVulkan :: MonadIO m => Window -> m Bool
+sdl2InitForVulkan (Window windowPtr) = liftIO do
+  ( 0 /= ) <$> [C.exp| bool { ImGui_ImplSDL2_InitForVulkan((SDL_Window*)$(void* windowPtr)) } |]