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.

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;