Add support for loop mode release (#1400)

Adds support for the new "sample mode release" introduced by Polyphone 2.4

src/rvoice/fluid_rvoice.c

@@ -422,6 +422,14 @@ fluid_rvoice_write(fluid_rvoice_t *voice, fluid_real_t *dsp_buf)
         voice->dsp.phase_incr = 1;
     }
 
+    /* loop mode release? if not in release, the voice is silent
+     * note: this intentionally processes the volenv before returning silence,
+     * since that's what polyphone does (PR #1400) */
+    if(voice->dsp.samplemode == FLUID_START_ON_RELEASE && fluid_adsr_env_get_section(&voice->envlfo.volenv) < FLUID_VOICE_ENVRELEASE)
+    {
+        return -1;
+    }
+
     /* voice is currently looping? */
     is_looping = voice->dsp.samplemode == FLUID_LOOP_DURING_RELEASE
                  || (voice->dsp.samplemode == FLUID_LOOP_UNTIL_RELEASE

src/rvoice/fluid_rvoice.h

@@ -45,7 +45,7 @@ enum fluid_loop
 {
     FLUID_UNLOOPED = 0,
     FLUID_LOOP_DURING_RELEASE = 1,
-    FLUID_NOTUSED = 2,
+    FLUID_START_ON_RELEASE = 2, /* this is a looping mode introduced by Polyphone, see #1398 for more info */
     FLUID_LOOP_UNTIL_RELEASE = 3
 };