Vulkan backend (#2)

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

dear-imgui.cabal

@@ -11,6 +11,14 @@ flag opengl
   manual:
     False
 
+flag vulkan
+  description:
+    Enable Vulkan backend.
+  default:
+    False
+  manual:
+    True
+
 flag sdl
   description:
     Enable SDL backend.
@@ -64,6 +72,27 @@ library
         extra-libraries:
           GL
 
+  if flag(vulkan)
+    exposed-modules:
+      DearImGui.Vulkan
+    other-modules:
+      DearImGui.Vulkan.Types
+    build-depends:
+        vulkan
+      , unliftio
+    cxx-sources:
+      imgui/backends/imgui_impl_vulkan.cpp
+    if os(windows)
+      extra-libraries:
+        vulkan-1
+    else
+      if os(darwin)
+        extra-libraries:
+          vulkan
+      else
+        pkgconfig-depends:
+          vulkan
+
   if flag(sdl)
     exposed-modules:
       DearImGui.SDL
@@ -83,6 +112,10 @@ library
       exposed-modules:
         DearImGui.SDL.OpenGL
 
+    if flag(vulkan)
+      exposed-modules:
+        DearImGui.SDL.Vulkan
+
 
 executable test
   main-is: Main.hs
@@ -97,3 +130,38 @@ executable readme
   default-language: Haskell2010
   build-depends: base, sdl2, gl, dear-imgui, managed
   ghc-options: -Wall
+
+executable vulkan
+  main-is: Main.hs
+  other-modules: Attachments, Backend, Input, Util
+  hs-source-dirs: examples/vulkan
+  default-language: Haskell2010
+  build-depends:
+      dear-imgui
+    , base
+        >= 4.13 && < 4.16
+    , bytestring
+        >= 0.10.10.0 && < 0.12
+    , containers
+       ^>= 0.6.2.1
+    , logging-effect
+       ^>= 1.3.12
+    , resourcet
+       ^>= 1.2.4.2
+    , sdl2
+       ^>= 2.5.3.0
+    , text-short
+       ^>= 0.1.3
+    , transformers
+       ^>= 0.5.6.2
+    , unliftio
+        >= 0.2.13 && < 0.2.15
+    , unliftio-core
+       ^>= 0.2.0.1
+    , vector
+       ^>= 0.12.1.2
+    , vulkan
+       ^>= 3.9
+    , vulkan-utils
+       ^>= 0.4.1
+  ghc-options: -Wall