openmw: build with its own openscenegraph fork

This gives speed improvement and fixes potential bugs.
For example: https://bugs.openmw.org/issues/3351
Updating our openscenegraph package would fix this too, but
it breaks some packages (i.e. simgear, bumping the version does
not help). Besides, fork has patches for better performance.

pkgs/games/openmw/default.nix

@@ -1,6 +1,20 @@
-{ stdenv, fetchFromGitHub, qt4, openscenegraph, mygui, bullet, ffmpeg, boost, cmake, SDL2, unshield, openal, pkgconfig }:
+{ stdenv, fetchFromGitHub, qt4, openscenegraph, mygui, bullet, ffmpeg, boost, cmake, SDL2, unshield, openal
+, giflib, pkgconfig }:
 
-stdenv.mkDerivation rec {
+let
+  openscenegraph_ = openscenegraph.override {
+    inherit ffmpeg giflib;
+    withApps = false;
+  };
+  openscenegraph__ = openscenegraph_.overrideDerivation (self: {
+    src = fetchFromGitHub {
+      owner = "OpenMW";
+      repo = "osg";
+      rev = "a72f43de6e1e4a8191643acb26c3e7138f833798";
+      sha256 = "04x2pjfrdz1kaj4i34zpzrmkk018jnr84rb6z646cz5fin3dapyh";
+    };
+  });
+in stdenv.mkDerivation rec {
   version = "0.38.0";
   name = "openmw-${version}";
 
@@ -13,7 +27,7 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
-  buildInputs = [ cmake boost ffmpeg qt4 bullet mygui openscenegraph SDL2 unshield openal pkgconfig ];
+  buildInputs = [ cmake boost ffmpeg qt4 bullet mygui openscenegraph__ SDL2 unshield openal pkgconfig ];
 
   meta = with stdenv.lib; {
     description = "An unofficial open source engine reimplementation of the game Morrowind";