add in cmake scripts for building SDL2

this gives us a great level of control over how we build SDL2, omitting parts we don't want, and including parts we do want, like libusb support!
windows sdl2 normally doesn't support libusb, but with some magic it can be built and linked in fairly easily, giving windows sdl2 libusb support.
libusb support in sdl2 means official GC adapter support! (resolves #1879)

linux build will be done in a later commit

.gitmodules

@@ -63,3 +63,10 @@
 	path = citra/citra
 	url = https://github.com/CasualPokePlayer/citra
 	branch = headless
+[submodule "ExternalProjects/SDL2/SDL"]
+	path = ExternalProjects/SDL2/SDL
+	url = https://github.com/libsdl-org/SDL.git
+	branch = SDL2
+[submodule "ExternalProjects/SDL2/libusb"]
+	path = ExternalProjects/SDL2/libusb
+	url = https://github.com/libusb/libusb.git

ExternalProjects/SDL2/.gitignore

@@ -0,0 +1 @@
+build/

ExternalProjects/SDL2/CMakeLists.txt

@@ -0,0 +1,140 @@
+cmake_minimum_required(VERSION 3.13 FATAL_ERROR)
+
+project(SDL2-BizHawk C CXX)
+
+# We only use a few subsystems, don't need to build the entire thing
+# Note that the defaults will be "enable if supported"
+# So sometimes it is better to simply not specify ON/OFF
+
+# set(SDL_ATOMIC ON) # Used in HIDAPI (and disabling it just makes it emulate atomics rather than use intrinsics)
+set(SDL_AUDIO OFF)
+set(SDL_VIDEO ON) # Used in mupen64plus + OpenGL/D3D9 display methods
+set(SDL_RENDER OFF)
+set(SDL_EVENTS ON) # Implied by SDL_VIDEO and SDL_JOYSTICK
+set(SDL_JOYSTICK ON) # Used for our SDL2 input adapter
+set(SDL_HAPTIC OFF)
+set(SDL_HIDAPI ON) # Not strictly necessary, but it gives more joystick support
+set(SDL_POWER OFF)
+# set(SDL_THREADS ON) # Used in some joystick backends
+# set(SDL_FILE ON) # Used in some joystick code (disabling this doesn't actually do anything on most platforms it seems...)
+# set(SDL_LOADSO ON) # Used (and sometimes required) in various subsystems
+# set(SDL_CPUINFO OFF) # Probably not actually needed (seems only used in SDL surface code?) but doesn't hurt much to add
+set(SDL_FILESYSTEM OFF)
+set(SDL_SENSOR OFF)
+set(SDL_LOCALE OFF)
+set(SDL_MISC OFF)
+
+# We're just building a shared library, so
+set(SDL2_DISABLE_SDL2MAIN ON)
+set(SDL2_DISABLE_INSTALL ON)
+set(SDL2_DISABLE_UNINSTALL ON)
+
+# Let's keep our build friendly to ALL x86-64 CPUs
+# set(SDL_SSE3 OFF)
+
+# Actually, SDL2 has a bug with clang-cl that makes SSE3 required
+# Annoying, but SSE3 shouldn't really hurt compatibility
+# (x86-64 was introduced in 2003, SSE3 was introduced in 2004)
+
+# We want to explicitly dynamically link against VC runtimes
+set(SDL_FORCE_STATIC_VCRT OFF)
+set(SDL_LIBC ON)
+
+# This is only used for IME stuff, we don't use that
+set(SDL_IBUS OFF)
+
+# We only currently support OpenGL (TODO: Do we want OGL ES support?)
+set(SDL_OPENGLES OFF)
+
+# Something only useful for audio stuff, don't include it
+set(SDL_LIBSAMPLERATE OFF)
+
+# We handle ld path finding with LD_LIBRARY_PATH for Linux
+set(SDL_RPATH OFF)
+
+# No Wayland support yet (TODO: Enable this if we ever get Wayland support)
+set(SDL_WAYLAND OFF)
+
+# We only need WGL for Windows and GLX for Linux
+set(SDL_DIRECTFB OFF)
+set(SDL_DUMMYVIDEO OFF)
+set(SDL_RPI OFF)
+set(SDL_RENDER_D3D OFF)
+set(SDL_VIVANTE OFF)
+set(SDL_VULKAN OFF)
+set(SDL_KMSDRM OFF)
+set(SDL_OFFSCREEN OFF)
+
+# libusb gives us support for even more joysticks (notably, the official GC Adapter!)
+set(SDL_HIDAPI_LIBUSB ON)
+
+# Windows builds don't normally ship libusb support, we'll do some magic to avoid that
+if(WIN32)
+	# This is enough to trick SDL to build with libusb support, really!
+	set(HAVE_LIBUSB ON)
+endif()
+
+# Mark this as a BizHawk build
+set(SDL_VENDOR_INFO "BizHawk" CACHE STRING "" FORCE)
+
+# We only want a shared library
+set(SDL_SHARED ON)
+set(SDL_STATIC OFF)
+set(SDL_TEST OFF)
+
+# We need the newer behavior for our changes to go through
+set(CMAKE_POLICY_DEFAULT_CMP0077 NEW)
+
+# Silence a warning when specifying INTERPROCEDURAL_OPTIMIZATION
+set(CMAKE_POLICY_DEFAULT_CMP0069 NEW)
+
+add_subdirectory(SDL)
+
+if(WIN32)
+	# We need to build libusb ourselves and statically link it
+	add_library(usb STATIC EXCLUDE_FROM_ALL
+		libusb/libusb/core.c
+		libusb/libusb/descriptor.c
+		libusb/libusb/hotplug.c
+		libusb/libusb/io.c
+		libusb/libusb/strerror.c
+		libusb/libusb/sync.c
+		libusb/libusb/os/threads_windows.c
+		libusb/libusb/os/windows_common.c
+		libusb/libusb/os/windows_usbdk.c
+		libusb/libusb/os/windows_winusb.c
+		libusb/libusb/os/events_windows.c
+	)
+
+	# Silence warnings with clang-cl
+	if(CMAKE_C_COMPILER_ID MATCHES "Clang")
+		target_compile_options(usb PRIVATE -w)
+	endif()
+
+	set_target_properties(usb PROPERTIES VERSION 1.0.26)
+	target_include_directories(usb BEFORE
+		PUBLIC libusb/libusb
+		PRIVATE libusb/msvc
+	)
+
+	target_link_libraries(SDL2 PRIVATE usb)
+endif()
+
+# ensure we have SDL2.dll / libSDL2.so
+set_target_properties(SDL2 PROPERTIES
+	NO_SONAME ON
+	OUTPUT_NAME "SDL2"
+)
+
+# Copy output to our dll folders
+# This requires a bit of magic (add_custom_command is not allowed for targets made with add_subdirectory)
+add_custom_target(SDL2-BizHawk ALL)
+add_dependencies(SDL2-BizHawk SDL2)
+add_custom_command(
+	TARGET SDL2-BizHawk
+	POST_BUILD
+	COMMAND ${CMAKE_COMMAND}
+	ARGS -E copy $<TARGET_FILE:SDL2> ${CMAKE_SOURCE_DIR}/../../Assets/dll
+	COMMAND ${CMAKE_COMMAND}
+	ARGS -E copy $<TARGET_FILE:SDL2> ${CMAKE_SOURCE_DIR}/../../output/dll
+)

ExternalProjects/SDL2/SDL

@@ -0,0 +1 @@
+Subproject commit 4339647d900bb8559ac3f6258166d21fe6d72a9a

ExternalProjects/SDL2/libusb

@@ -0,0 +1 @@
+Subproject commit 4239bc3a50014b8e6a5a2a59df1fff3b7469543b