SDL2_mixer: add "dev" output

`SDL2_mixer` is a a popular game dependency. Split it's `dev` output to
separate store path to reduce runtime closure size.

While at it amend a few games's include paths.

pkgs/development/libraries/SDL2_mixer/default.nix

···
     timidity
   ];
 
+  outputs = [ "out" "dev" ];
+
   # fix default path to timidity.cfg so MIDI files could be played
   postPatch = ''
     substituteInPlace timidity/options.h \

pkgs/games/eduke32/default.nix

···
     done
   '';
 
-  NIX_CFLAGS_COMPILE = "-I${SDL2.dev}/include/SDL2 -I${SDL2_mixer}/include/SDL2";
+  NIX_CFLAGS_COMPILE = "-I${lib.getDev SDL2}/include/SDL2 -I${lib.getDev SDL2_mixer}/include/SDL2";
 
   makeFlags = [
     "SDLCONFIG=${SDL2}/bin/sdl2-config"

pkgs/games/ivan/default.nix

···
   cmakeFlags = ["-DCMAKE_CXX_FLAGS=-DWIZARD"];
 
   # Help CMake find SDL_mixer.h
-  NIX_CFLAGS_COMPILE = "-I${SDL2_mixer}/include/SDL2";
+  NIX_CFLAGS_COMPILE = "-I${lib.getDev SDL2_mixer}/include/SDL2";
 
   # Create "ivan.desktop" file
   ivanDesktop = makeDesktopItem {

pkgs/games/srb2/default.nix

···
     "-DSRB2_ASSET_DIRECTORY=/build/source/assets"
     "-DGME_INCLUDE_DIR=${game-music-emu}/include"
     "-DOPENMPT_INCLUDE_DIR=${libopenmpt.dev}/include"
-    "-DSDL2_MIXER_INCLUDE_DIR=${SDL2_mixer}/include/SDL2"
-    "-DSDL2_INCLUDE_DIR=${SDL2.dev}/include/SDL2"
+    "-DSDL2_MIXER_INCLUDE_DIR=${lib.getDev SDL2_mixer}/include/SDL2"
+    "-DSDL2_INCLUDE_DIR=${lib.getDev SDL2}/include/SDL2"
   ];
 
   patches = [

pkgs/games/srb2kart/default.nix

···
   cmakeFlags = [
     #"-DSRB2_ASSET_DIRECTORY=/build/source/assets"
     "-DGME_INCLUDE_DIR=${game-music-emu}/include"
-    "-DSDL2_MIXER_INCLUDE_DIR=${SDL2_mixer}/include/SDL2"
-    "-DSDL2_INCLUDE_DIR=${SDL2.dev}/include/SDL2"
+    "-DSDL2_MIXER_INCLUDE_DIR=${lib.getDev SDL2_mixer}/include/SDL2"
+    "-DSDL2_INCLUDE_DIR=${lib.getDev SDL2}/include/SDL2"
   ];
 
   patches = [