Fix: Allow full sample range for 16-bit audio.

Clamping each sample value to half the available range could cause
unnecessary premature clipping with lots of sounds playing. This change
does not affect the actual volume level.

src/mixer.cpp

@@ -46,7 +46,7 @@ static MxStreamCallback _music_stream = nullptr;
  * stops overflowing when too many sounds are played at the same time, which
  * causes an even worse sound quality.
  */
-static const int MAX_VOLUME = 128 * 128;
+static const int MAX_VOLUME = 32767;
 
 /**
  * Perform the rate conversion between the input and output.