SDL: enable PulseAudio support by default

On a system with PulseAudio enabled, applications which use SDL work
poorly or don't work at all e.g. MLT, Kdenlive, Shotcut, Blender and
Hedgewars. Rather than enabling it per application as we run into
issues, I think it's better to enable support for it by default.

pkgs/top-level/all-packages.nix

···
 
   mlt-qt4 = callPackage ../development/libraries/mlt {
     qt = qt4;
-    SDL = SDL_pulseaudio;
   };
 
   mlt-qt5 = callPackage ../development/libraries/mlt {
     qt = qt5;
-    SDL = SDL_pulseaudio;
   };
 
   movit = callPackage ../development/libraries/movit { };
···
     openglSupport = mesaSupported;
     alsaSupport = (!stdenv.isDarwin);
     x11Support = true;
-    pulseaudioSupport = stdenv.isDarwin; # better go through ALSA
+    pulseaudioSupport = true;
 
     # resolve the unrecognized -fpascal-strings option error
     stdenv = if stdenv.isDarwin
       then clangStdenv
       else stdenv;
   };
-
-  # Fixes major problems with choppy sound in MLT / Kdenlive / Shotcut
-  SDL_pulseaudio = SDL.override { pulseaudioSupport = true; };
 
   SDL_gfx = callPackage ../development/libraries/SDL_gfx { };
 
···
   hedgewars = callPackage ../games/hedgewars {
     inherit (haskellPackages) ghc network vector utf8String bytestringShow
       random hslogger dataenc;
-    SDL = SDL_pulseaudio;
   };
 
   hexen = callPackage ../games/hexen { };