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.

.github/workflows/switch.yml

@@ -5,6 +5,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     container: devkitpro/devkita64:latest
+    name: Nintendo Switch SDL2
 
     steps:
     - uses: actions/checkout@v6
@@ -15,7 +16,8 @@ jobs:
       run: |
         mkdir -p out/switch
         cd out/switch
-        /opt/devkitpro/portlibs/switch/bin/aarch64-none-elf-cmake -DCMAKE_BUILD_TYPE=Release ../../
+        /opt/devkitpro/portlibs/switch/bin/aarch64-none-elf-cmake \
+          -DCMAKE_BUILD_TYPE=Release -DSYSTEM3_SDL_VERSION=2 ../../
         make -j4
         mkdir system3
         mv system3.* system3/