make mic ducking use strength the same way in Auto & Man mode

Previously, engine/enginetalkoverducking returned opposite values
for 'strength' in Auto and Manual mode. This commit removes the
inversion from Auto mode, thus the Strength knob always affects the
music volume in the same way with both modes:
* fully left: music muted completely
* fully right: music volume unchanged

src/engine/enginesidechaincompressor.cpp

@@ -6,7 +6,7 @@ EngineSideChainCompressor::EngineSideChainCompressor(const char* group)
         : m_compressRatio(0.0),
           m_bAboveThreshold(false),
           m_threshold(1.0),
-          m_strength(0.0),
+          m_strength(1.0),
           m_attackTime(0),
           m_decayTime(0),
           m_attackPerFrame(0.0),
@@ -54,28 +54,28 @@ void EngineSideChainCompressor::processKey(const CSAMPLE* pIn, const int iBuffer
 
 double EngineSideChainCompressor::calculateCompressedGain(int frames) {
     if (m_bAboveThreshold) {
-        if (m_compressRatio < m_strength) {
-            m_compressRatio += m_attackPerFrame * frames;
-            if (m_compressRatio > m_strength) {
+        if (m_compressRatio > m_strength) {
+            m_compressRatio -= m_attackPerFrame * frames;
+            if (m_compressRatio < m_strength) {
                 // If we overshot, clamp.
                 m_compressRatio = m_strength;
             }
-        } else if (m_compressRatio > m_strength) {
+        } else if (m_compressRatio < m_strength) {
             // If the strength param was changed, we might be compressing too much.
-            m_compressRatio -= m_decayPerFrame * frames;
+            m_compressRatio += m_decayPerFrame * frames;
         }
     } else {
         if (m_compressRatio > 0) {
-            m_compressRatio -= m_decayPerFrame * frames;
-            if (m_compressRatio < 0) {
+            m_compressRatio += m_decayPerFrame * frames;
+            if (m_compressRatio > 1) {
                 // If we overshot, clamp.
-                m_compressRatio = 0;
+                m_compressRatio = 1;
             }
         } else if (m_compressRatio < 0) {
             // Complain loudly.
             qWarning() << "Programming error, below-zero compression detected.";
             m_compressRatio += m_attackPerFrame * frames;
         }
     }
-    return (1. - m_compressRatio);
+    return m_compressRatio;
 }