Implement ESS activity flags and improve the timed IRQ triggering via SB DSP commands F2 and F3, fixes ESSCFG 2.01C and the LBA 2 ES(1)688 driver.

src/include/86box/snd_sb.h

@@ -187,6 +187,8 @@ typedef struct sb_t {
     uint8_t pnp_rom[512];
 
     uint16_t opl_pnp_addr;
+
+    uint16_t midi_addr;
     uint16_t gameport_addr;
 
     void   *opl_mixer;

src/include/86box/snd_sb_dsp.h

@@ -91,6 +91,8 @@ typedef struct sb_dsp_t {
     uint8_t sbref;
     int8_t  sbstep;
 
+    uint8_t activity;
+
     int sbdacpos;
 
     int sbleftright;