Add SDL3 support alongside SDL2

Add optional SDL3 support while keeping SDL2 as the default. SDL3 builds
must be selected explicitly with the SYSTEM3_SDL_VERSION CMake option.

Centralize most SDL2/SDL3 API differences behind compatibility interfaces.
Audio playback uses a dedicated SDL3 backend based on SDL3 audio streams,
implemented separately in mako_sdl3.cpp.

Android and Nintendo Switch remain on SDL2 for now.

src/sys/game_id.cpp

@@ -1,5 +1,5 @@
 #include <string.h>
-#include <SDL.h>
+#include "sdl_compat.h"
 #include "game_id.h"
 #include "fileio.h"
 #include "config.h"