Mono process for the crossfade envelope

The right channel should be blank at this point of the process so you only need to apply the envelope to the left channel

src/sfizz/Voice.cpp

@@ -293,7 +293,7 @@ void sfz::Voice::processMono(AudioSpan<float> buffer) noexcept
 
     // Crossfade envelope
     crossfadeEnvelope.getBlock(span1);
-    buffer.applyGain(span1);
+    applyGain<float>(span1, leftBuffer);
 
     // Volume envelope
     volumeEnvelope.getBlock(span1);