audio: Assigning a device channel map to an audio stream was quietly failing.

Fixes #10317.

src/audio/SDL_audio.c

@@ -251,7 +251,7 @@ static void UpdateAudioStreamFormatsPhysical(SDL_AudioDevice *device)
                 // SDL_SetAudioStreamFormat does a ton of validation just to memcpy an audiospec.
                 SDL_LockMutex(stream->lock);
                 SDL_copyp(&stream->dst_spec, &spec);
-                SDL_SetAudioStreamOutputChannelMap(stream, device->chmap, spec.channels);  // this should be fast for normal cases, though!
+                SetAudioStreamChannelMap(stream, &stream->dst_spec, &stream->dst_chmap, device->chmap, spec.channels, -1);  // this should be fast for normal cases, though!
                 SDL_UnlockMutex(stream->lock);
             }
         }

src/audio/SDL_audiocvt.c

@@ -584,7 +584,7 @@ int SDL_SetAudioStreamFormat(SDL_AudioStream *stream, const SDL_AudioSpec *src_s
     return 0;
 }
 
-static int SetAudioStreamChannelMap(SDL_AudioStream *stream, const SDL_AudioSpec *spec, int **stream_chmap, const int *chmap, int channels, SDL_bool isinput)
+int SetAudioStreamChannelMap(SDL_AudioStream *stream, const SDL_AudioSpec *spec, int **stream_chmap, const int *chmap, int channels, int isinput)
 {
     if (!stream) {
         return SDL_InvalidParamError("stream");
@@ -602,7 +602,7 @@ static int SetAudioStreamChannelMap(SDL_AudioStream *stream, const SDL_AudioSpec
         // already have this map, don't allocate/copy it again.
     } else if (SDL_ChannelMapIsBogus(chmap, channels)) {
         retval = SDL_SetError("Invalid channel mapping");
-    } else if (stream->bound_device && (!!isinput == !!stream->bound_device->physical_device->recording)) {
+    } else if ((isinput != -1) && stream->bound_device && (!!isinput == !!stream->bound_device->physical_device->recording)) {
         // quietly refuse to change the format of the end currently bound to a device.
     } else {
         if (SDL_ChannelMapIsDefault(chmap, channels)) {

src/audio/SDL_sysaudio.h

@@ -137,6 +137,10 @@ extern void OnAudioStreamDestroy(SDL_AudioStream *stream);
 // This just lets audio playback apply logical device gain at the same time as audiostream gain, so it's one multiplication instead of thousands.
 extern int SDL_GetAudioStreamDataAdjustGain(SDL_AudioStream *stream, void *voidbuf, int len, float extra_gain);
 
+// This is the bulk of `SDL_SetAudioStream*putChannelMap`'s work, but it lets you skip the check about changing the device end of a stream if isinput==-1.
+extern int SetAudioStreamChannelMap(SDL_AudioStream *stream, const SDL_AudioSpec *spec, int **stream_chmap, const int *chmap, int channels, int isinput);
+
+
 typedef struct SDL_AudioDriverImpl
 {
     void (*DetectDevices)(SDL_AudioDevice **default_playback, SDL_AudioDevice **default_recording);