SDL: Audio: convert 5.1 to 7.1, use right-surround for r-front and r-back (see #4104)

From 21349901ea250483600cbe4771d865cc06f6b538 Mon Sep 17 00:00:00 2001
From: Sylvain <[EMAIL REDACTED]>
Date: Tue, 27 Apr 2021 10:57:48 +0200
Subject: [PATCH] Audio: convert 5.1 to 7.1, use right-surround for r-front and
 r-back (see #4104)

---
 src/audio/SDL_audiocvt.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/audio/SDL_audiocvt.c b/src/audio/SDL_audiocvt.c
index f6efb1381..ca307f84e 100644
--- a/src/audio/SDL_audiocvt.c
+++ b/src/audio/SDL_audiocvt.c
@@ -353,9 +353,9 @@ SDL_Convert51To71(SDL_AudioCVT * cvt, SDL_AudioFormat format)
         rs = (rf + rb) * 0.5f;
         /* !!! FIXME: these four may clip */
         lf += lf - ls;
-        rf += rf - ls;
+        rf += rf - rs;
         lb += lb - ls;
-        rb += rb - ls;
+        rb += rb - rs;
         dst[3] = src[3];  /* LFE */
         dst[2] = src[2];  /* FC */
         dst[7] = rs; /* SR */