Merge branch 'next' into lua-sector-lines

.travis.yml

CMakeLists.txt

cmake/Modules/FindSDL2.cmake

cmake/Modules/FindSDL2_main.cmake

cmake/Modules/FindSDL2_mixer.cmake

cmake/Modules/LibFindMacros.cmake

src/doomdef.h

src/p_inter.c

src/p_map.c

src/p_user.c

src/sdl/CMakeLists.txt

src/sdl/i_system.c

src/sdl/i_video.c

src/sdl/mixer_sound.c

src/sdl/sdl_sound.c

src/win32/win_sys.c