Add SDL3 support alongside SDL2

Centralize the differences between SDL2 and SDL3 behind compatibility
interfaces and select the appropriate implementations with the
SYSTEM3_SDL_VERSION CMake option. The default is SDL2 except on Android,
which supports only SDL3.

.github/workflows/android.yml

@@ -4,6 +4,7 @@ on: [push, pull_request]
 jobs:
   build:
     runs-on: ubuntu-latest
+    name: Android SDL3
 
     steps:
     - uses: actions/checkout@v6
@@ -26,5 +27,6 @@ jobs:
     - name: Upload artifacts
       uses: actions/upload-artifact@v7
       with:
+        name: system3-android-sdl3
         path: android/app/build/outputs/apk/release/app-release.apk
         archive: false