Add support for SDL2 Renderer backend (#193)

* Add DearImGui.Raw.framerate
* Add DearImGui.withCloseableWindow
* Closes #191: Add support for SDL2 Renderer backend
* Add sdl-renderer flag to protect against older SDL versions that do not have SDL_RenderGeometry

examples/sdl/Renderer.hs

@@ -0,0 +1,146 @@
+{-# language BlockArguments #-}
+{-# language LambdaCase #-}
+{-# language OverloadedStrings #-}
+
+-- | Port of [example_sdl2_sdlrenderer2](https://github.com/ocornut/imgui/blob/54c1bdecebf3c9bb9259c07c5f5666bb4bd5c3ea/examples/example_sdl2_sdlrenderer2/main.cpp).
+--
+-- Minor differences:
+-- - No changing of the clear color via @ImGui::ColorEdit3@ as a Haskell binding
+-- doesn't yet exist for this function.
+-- - No high DPI renderer scaling as this seems to be in flux [upstream](https://github.com/ocornut/imgui/issues/6065)
+
+module Main ( main ) where
+
+import Control.Exception (bracket, bracket_)
+import Control.Monad.IO.Class (MonadIO(liftIO))
+import Control.Monad.Managed (managed, managed_, runManaged)
+import Data.IORef (IORef, newIORef)
+import Data.Text (pack)
+import DearImGui
+import DearImGui.SDL (pollEventWithImGui, sdl2NewFrame, sdl2Shutdown)
+import DearImGui.SDL.Renderer
+  ( sdl2InitForSDLRenderer, sdlRendererInit, sdlRendererNewFrame, sdlRendererRenderDrawData
+  , sdlRendererShutdown
+  )
+import SDL (V4(V4), ($=), ($~), get)
+import Text.Printf (printf)
+import qualified SDL
+
+
+main :: IO ()
+main = do
+  -- Initialize SDL2
+  SDL.initializeAll
+
+  runManaged do
+    -- Create a window using SDL2
+    window <- do
+      let title = "ImGui + SDL2 Renderer"
+      let config = SDL.defaultWindow
+            { SDL.windowInitialSize = SDL.V2 1280 720
+            , SDL.windowResizable = True
+            , SDL.windowPosition = SDL.Centered
+            }
+      managed $ bracket (SDL.createWindow title config) SDL.destroyWindow
+
+    -- Create an SDL2 renderer
+    renderer <- managed do
+      bracket
+        (SDL.createRenderer window (-1) SDL.defaultRenderer)
+        SDL.destroyRenderer
+
+    -- Create an ImGui context
+    _ <- managed $ bracket createContext destroyContext
+
+    -- Initialize ImGui's SDL2 backend
+    _ <- managed_ do
+      bracket_ (sdl2InitForSDLRenderer window renderer) sdl2Shutdown
+
+    -- Initialize ImGui's SDL2 renderer backend
+    _ <- managed_ $ bracket_ (sdlRendererInit renderer) sdlRendererShutdown
+
+    liftIO $ mainLoop renderer
+
+
+mainLoop :: SDL.Renderer -> IO ()
+mainLoop renderer = do
+  refs <- newRefs
+  go refs
+  where
+  go refs = unlessQuit do
+    -- Tell ImGui we're starting a new frame
+    sdlRendererNewFrame
+    sdl2NewFrame
+    newFrame
+
+    -- Show the ImGui demo window
+    get (refsShowDemoWindow refs) >>= \case
+      False -> pure ()
+      True -> showDemoWindow
+
+    withWindowOpen "Hello, world!" do
+      text "This is some useful text."
+      _ <- checkbox "Demo Window" $ refsShowDemoWindow refs
+      _ <- checkbox "Another Window" $ refsShowAnotherWindow refs
+      _ <- sliderFloat "float" (refsFloat refs) 0 1
+
+      button "Button" >>= \case
+        False -> pure ()
+        True -> refsCounter refs $~ succ
+      sameLine
+      counter <- get $ refsCounter refs
+      text $ "counter = " <> pack (show counter)
+
+      fr <- framerate
+      text
+        $ pack
+        $ printf "Application average %.3f ms/frame (%.1f FPS)" (1000 / fr) fr
+
+    get (refsShowAnotherWindow refs) >>= \case
+      False -> pure ()
+      True ->
+        withCloseableWindow "Another Window" (refsShowAnotherWindow refs) do
+          text "Hello from another window!"
+          button "Close Me" >>= \case
+            False -> pure ()
+            True -> refsShowAnotherWindow refs $= False
+
+    -- Render
+    SDL.rendererDrawColor renderer $= V4 0 0 0 255
+    SDL.clear renderer
+    render
+    sdlRendererRenderDrawData =<< getDrawData
+    SDL.present renderer
+
+    go refs
+
+  -- Process the event loop
+  unlessQuit action = do
+    shouldQuit <- checkEvents
+    if shouldQuit then pure () else action
+
+  checkEvents = do
+    pollEventWithImGui >>= \case
+      Nothing ->
+        return False
+      Just event ->
+        (isQuit event ||) <$> checkEvents
+
+  isQuit event =
+    SDL.eventPayload event == SDL.QuitEvent
+
+
+data Refs = Refs
+  { refsShowDemoWindow :: IORef Bool
+  , refsShowAnotherWindow :: IORef Bool
+  , refsFloat :: IORef Float
+  , refsCounter :: IORef Int
+  }
+
+newRefs :: IO Refs
+newRefs =
+  Refs
+    <$> newIORef True
+    <*> newIORef False
+    <*> newIORef 0
+    <*> newIORef 0