SDL: audio: Fix audio stream callback calculations when future buffer has space.

SDL Commits
1 
From 0714da37a4f4edf37a2d3845e2aed50d1df437d4 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 8 Aug 2023 21:40:23 -0400
Subject: [PATCH] audio: Fix audio stream callback calculations when future
 buffer has space.

We were subtracting backwards.  :/
---
 src/audio/SDL_audiocvt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/audio/SDL_audiocvt.c b/src/audio/SDL_audiocvt.c
index 0098504e22d3..f1517ae6c188 100644
--- a/src/audio/SDL_audiocvt.c
+++ b/src/audio/SDL_audiocvt.c
@@ -1038,7 +1038,7 @@ int SDL_GetAudioStreamData(SDL_AudioStream *stream, void *voidbuf, int len)
             // calculate difference in dataset size after resampling. Use a Uint64 so the multiplication doesn't overflow.
             approx_request = (int) (size_t) ((((Uint64) approx_request) * stream->src_spec.freq) / stream->dst_spec.freq);
             if (!stream->flushed) {  // do we need to fill the future buffer to accomodate this, too?
-                approx_request += stream->future_buffer_filled_frames - stream->resampler_padding_frames;
+                approx_request += stream->resampler_padding_frames - stream->future_buffer_filled_frames;
             }
         }