GB Audio: Improve "zombie mode" emulation in CGB mode (fixes #2029)

CHANGES

@@ -11,6 +11,7 @@ Emulation fixes:
  - GB Audio: Fix channels 1/2 staying muted if restarted after long silence
  - GB Audio: Fix channel 1 restarting if sweep applies after stop (fixes mgba.io/i/2965)
  - GB Audio: Fix restarting envelope when writing to register (fixes mgba.io/i/3067)
+ - GB Audio: Improve "zombie mode" emulation in CGB mode (fixes mgba.io/i/2029)
  - GB I/O: Read back proper SVBK value after writing 0 (fixes mgba.io/i/2921)
  - GB I/O: Fix STAT writing IRQ trigger conditions (fixes mgba.io/i/2501)
  - GB Serialize: Add missing Pocket Cam state to savestates

src/gb/audio.c

@@ -911,12 +911,25 @@ void _writeDuty(struct GBAudioEnvelope* envelope, uint8_t value) {
 }
 
 bool _writeEnvelope(struct GBAudioEnvelope* envelope, uint8_t value, enum GBAudioStyle style) {
+	bool oldDirection = envelope->direction;
 	envelope->stepTime = GBAudioRegisterSweepGetStepTime(value);
 	envelope->direction = GBAudioRegisterSweepGetDirection(value);
 	envelope->initialVolume = GBAudioRegisterSweepGetInitialVolume(value);
-	if (style == GB_AUDIO_DMG && !envelope->stepTime) {
+	if (!envelope->stepTime) {
 		// TODO: Improve "zombie" mode
-		++envelope->currentVolume;
+		if (style == GB_AUDIO_DMG) {
+			++envelope->currentVolume;
+		} else if (style == GB_AUDIO_CGB) {
+			if (envelope->direction == oldDirection) {
+				if (envelope->direction) {
+					++envelope->currentVolume;
+				} else {
+					envelope->currentVolume += 2;
+				}
+			} else {
+				envelope->currentVolume = 0;
+			}
+		}
 		envelope->currentVolume &= 0xF;
 	}
 	_updateEnvelopeDead(envelope);