src/Nxt/Graphics.hs: fix against opengl-2.9

> src/Nxt/Graphics.hs:91:5:
>     No instance for (TwoDimensionalTextureTarget (Maybe a0))
>       arising from a use of `texImage2D'

Reported by kini.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

src/Nxt/Graphics.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 module Nxt.Graphics
     (begin,
      end,
@@ -88,7 +89,13 @@ loadTexture textureFilePath = do
     surfacePixels <- surfaceGetPixels surface
 
     let pixelData = PixelData RGBA UnsignedByte surfacePixels
-    texImage2D Nothing NoProxy 0 RGBA' surfaceSize 0 pixelData
+    texImage2D
+#if MIN_VERSION_OpenGL(2,9,0)
+          Texture2D
+#else
+          Nothing
+#endif
+          NoProxy 0 RGBA' surfaceSize 0 pixelData
 
     freeSurface surface