Add helpers to count playing voices in polyphony groups

src/sfizz/PolyphonyGroup.cpp

@@ -16,3 +16,10 @@ void sfz::PolyphonyGroup::removeVoice(const Voice* voice) noexcept
 {
     swapAndPopFirst(voices, [voice](const Voice* v) { return v == voice; });
 }
+
+unsigned sfz::PolyphonyGroup::numPlayingVoices() const noexcept
+{
+    return absl::c_count_if(voices, [](const Voice* v) {
+        return !v->releasedOrFree();
+    });
+}

src/sfizz/PolyphonyGroup.h

@@ -40,6 +40,10 @@ public:
      * @return unsigned
      */
     unsigned getPolyphonyLimit() const noexcept { return polyphonyLimit; }
+    /**
+     * @brief Returns the number of playing (unreleased) voices
+     */
+    unsigned numPlayingVoices() const noexcept;
     /**
      * @brief Get the active voices
      *

src/sfizz/RegionSet.cpp

@@ -46,3 +46,10 @@ void sfz::RegionSet::removeVoiceFromHierarchy(const Region* region, const Voice*
         parent = parent->getParent();
     }
 }
+
+unsigned sfz::RegionSet::numPlayingVoices() const noexcept
+{
+    return absl::c_count_if(voices, [](const Voice* v) {
+        return !v->releasedOrFree();
+    });
+}

src/sfizz/RegionSet.h

@@ -79,6 +79,10 @@ public:
      * @param parent
      */
     void setParent(RegionSet* parent) noexcept { this->parent = parent; }
+    /**
+     * @brief Returns the number of playing (unreleased) voices
+     */
+    unsigned numPlayingVoices() const noexcept;
     /**
      * @brief Get the active voices
      *