Fix SFX and MIDI music not playing on Android

snd_start_digital() opened two separate SDL audio devices - a raw one
for cutscene/audiolog audio and a second via Mix_OpenAudio() for
SFX/MIDI - but Android's audio backends only ever allow one open
non-capture device, so the second open always failed silently.

- Consolidate onto a single Mix_OpenAudio() device; cutscene/audiolog
  PCM is now mixed into its output via Mix_SetPostMix() instead of a
  dedicated device (16-android-audio-single-device.patch)
- Add SDL_GetError() to the failure log so a future open failure is
  actually diagnosable (15-android-audio-error-detail.patch)
- Make NativeMidiInit() report failure on platforms with no real
  implementation (Android included), instead of silently claiming
  success without opening anything (17-android-native-midi-unsupported.patch)
- Strengthen the OpenSL ES driver hint to SDL_HINT_OVERRIDE and log the
  active audio driver as a standing diagnostic (05-android-audio-driver.patch)

android/engine-patches/05-android-audio-driver.patch

@@ -1,6 +1,6 @@
 --- a/src/MacSrc/Shock.c
 +++ b/src/MacSrc/Shock.c
-@@ -162,6 +162,17 @@
+@@ -162,9 +162,26 @@
  void InitSDL() {
      SDL_SetHint(SDL_HINT_NO_SIGNAL_HANDLERS, "1");
      SDL_SetHint(SDL_HINT_RENDER_DRIVER, "opengl");
@@ -12,9 +12,18 @@
 +    // src/audio/aaudio/SDL_aaudio.c). SDLSound.c opens two: one directly via
 +    // SDL_OpenAudioDevice() for cutscene audio, one via Mix_OpenAudio() for
 +    // SFX/MIDI. The older OpenSL ES backend has no such limitation, so force
-+    // it instead of AAudio.
-+    SDL_SetHint(SDL_HINT_AUDIODRIVER, "openslES");
++    // it instead of AAudio. Uses SDL_HINT_OVERRIDE priority so nothing set
++    // earlier (or by SDL/the platform) can take precedence over it.
++    SDL_SetHintWithPriority(SDL_HINT_AUDIODRIVER, "openslES", SDL_HINT_OVERRIDE);
 +#endif
      if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER | SDL_INIT_AUDIO) < 0) {
          DEBUG("%s: Init failed", __FUNCTION__);
      }
++#ifdef __ANDROID__
++    // Standing diagnostic: reports which audio driver SDL actually selected,
++    // to confirm the openslES override above took effect on-device.
++    INFO("SDL audio driver: %s", SDL_GetCurrentAudioDriver());
++#endif
+ 
+     // TODO: figure out some universal set of settings that work...
+ #ifdef __ANDROID__