SDL: audio: Fix postmix state when migrating to new default devices.

SDL Commits
1 
From 8fa0746d4af9a354160f724f8d3ca1c439528e30 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Wed, 15 Nov 2023 14:10:29 -0500
Subject: [PATCH] audio: Fix postmix state when migrating to new default
 devices.

Otherwise buffers might not be allocated, etc.
---
 src/audio/SDL_audio.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/audio/SDL_audio.c b/src/audio/SDL_audio.c
index 6bbd666c7604..4f13b54a8a33 100644
--- a/src/audio/SDL_audio.c
+++ b/src/audio/SDL_audio.c
@@ -2065,6 +2065,8 @@ void SDL_DefaultAudioDeviceChanged(SDL_AudioDevice *new_default_device)
                 RefPhysicalAudioDevice(new_default_device);
                 UnrefPhysicalAudioDevice(current_default_device);
 
+                SDL_SetAudioPostmixCallback(logdev->instance_id, logdev->postmix, logdev->postmix_userdata);
+
                 SDL_PendingAudioDeviceEvent *p;
 
                 // Queue an event for each logical device we moved.