s/audio/sample: increase max samples to 20

Observed in #281. When Lara shoots Uzis and a mutant simultaneously
explodes, the guns can just go silent.

src/specific/s_audio_sample.c

@@ -10,7 +10,7 @@
 #include <libavformat/avformat.h>
 
 #define MAX_SAMPLES 1000
-#define MAX_ACTIVE_SAMPLES 10
+#define MAX_ACTIVE_SAMPLES 20
 
 typedef struct AUDIO_SAMPLE {
     float *sample_data;