Fix SDL audio playback with 16-bit stereo sound

My commit last month "Fix SDL audio playback with surround sound" broke
16-bit stereo sound. S_TransferStereo16() still assumed that dma.samples
was a power of two. I also cleaned up code related to the previously
mentioned commit.

code/client/snd_local.h

@@ -65,6 +65,7 @@ typedef struct sfx_s {
 typedef struct {
 	int			channels;
 	int			samples;				// mono samples in buffer
+	int			fullsamples;			// samples with all channels in buffer (samples divided by channels)
 	int			submission_chunk;		// don't mix less than this #
 	int			samplebits;
 	int			isfloat;