audio: don't cast to double in SDL_ConvertStereoToMono().

It's expensive and (hopefully) unnecessary. If this becomes an overflow
problem, we could multiply both values by 0.5f before adding them, but let's
see if we can get by without the extra multiplication first.

--HG--
extra : rebase_source : b7b47e961eb974510e133882548ea36b40f6d7e3

src/audio/SDL_audiocvt.c

@@ -41,7 +41,7 @@ SDL_ConvertStereoToMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
     SDL_assert(format == AUDIO_F32SYS);
 
     for (i = cvt->len_cvt / 8; i; --i, src += 2) {
-        *(dst++) = (float) ((((double) src[0]) + ((double) src[1])) * 0.5);
+        *(dst++) = (src[0] + src[1]) * 0.5f;
     }
 
     cvt->len_cvt /= 2;