Add support for GLFW (#26)

Co-authored-by: Oliver Charles <ollie@ocharles.org.uk>

examples/glfw/Main.hs

@@ -0,0 +1,82 @@
+{-# language BlockArguments #-}
+{-# language LambdaCase #-}
+{-# language OverloadedStrings #-}
+
+module Main ( main ) where
+
+import Control.Exception
+import Control.Monad
+import Control.Monad.IO.Class
+import Control.Monad.Managed
+import DearImGui
+import DearImGui.OpenGL
+import DearImGui.GLFW
+import DearImGui.GLFW.OpenGL
+import Graphics.GL
+import Graphics.UI.GLFW (Window)
+import qualified Graphics.UI.GLFW as GLFW
+
+main :: IO ()
+main = do
+  initialised <- GLFW.init
+  unless initialised $ error "GLFW init failed"
+
+  runManaged $ do
+    mwin <- managed $ bracket
+      (GLFW.createWindow 800 600 "Hello, Dear ImGui!" Nothing Nothing)
+      (maybe (return ()) GLFW.destroyWindow)
+    case mwin of
+      Just win -> do
+        liftIO $ do
+          GLFW.makeContextCurrent (Just win)
+          GLFW.swapInterval 1
+
+        -- Create an ImGui context
+        _ <- managed $ bracket createContext destroyContext
+
+        -- Initialize ImGui's GLFW backend
+        _ <- managed_ $ bracket_ (glfwInitForOpenGL win True) glfwShutdown
+
+        -- Initialize ImGui's OpenGL backend
+        _ <- managed_ $ bracket_ openGL2Init openGL2Shutdown
+
+        liftIO $ mainLoop win
+      Nothing -> do
+        error "GLFW createWindow failed"
+
+  GLFW.terminate
+
+mainLoop :: Window -> IO ()
+mainLoop win = do
+  -- Process the event loop
+  GLFW.pollEvents
+  close <- GLFW.windowShouldClose win
+  unless close do
+
+    -- Tell ImGui we're starting a new frame
+    openGL2NewFrame
+    glfwNewFrame
+    newFrame
+
+    -- Build the GUI
+    bracket_ (begin "Hello, ImGui!") end do
+      -- Add a text widget
+      text "Hello, ImGui!"
+
+      -- Add a button widget, and call 'putStrLn' when it's clicked
+      button "Clickety Click" >>= \case
+        False -> return ()
+        True  -> putStrLn "Ow!"
+
+    -- Show the ImGui demo window
+    showDemoWindow
+
+    -- Render
+    glClear GL_COLOR_BUFFER_BIT
+
+    render
+    openGL2RenderDrawData =<< getDrawData
+
+    GLFW.swapBuffers win
+
+    mainLoop win