Avoid per-line gains for the secondary early reflections

Similar to the first reflections/input delay, there may still be enough spatial
coeherence and cause direction-dependent attenuation. Since the first line
provides a 0-delay path from the early reflections to the late reverb, use the
average instead of the minimum delay. Not sure if using the decay time is the
best idea, but since these are additional reflections after more room
interaction, it doesn't seem completely wrong.

Author: kcat

alc/effects/reverb.cpp

@@ -401,7 +401,7 @@ struct EarlyReflections {
      */
     DelayLineU Delay;
     std::array<size_t,NUM_LINES> Offset{};
-    std::array<float,NUM_LINES> Coeff{};
+    float Coeff{};
 
     /* The gain for each output channel based on 3D panning. */
     struct OutGains {
@@ -926,10 +926,15 @@ void EarlyReflections::updateLines(const float density_mult, const float diffusi
         /* Calculate the delay length of each delay line. */
         length = EARLY_LINE_LENGTHS[i] * density_mult;
         Offset[i] = float2uint(length * frequency);
-
-        /* Calculate the gain (coefficient) for each line. */
-        Coeff[i] = CalcDecayCoeff(length, decayTime);
     }
+
+    /* Calculate the gain (coefficient) for the secondary reflections based on
+     * the average delay and decay time.
+     */
+    const auto length = std::reduce(EARLY_LINE_LENGTHS.begin(), EARLY_LINE_LENGTHS.end(), 0.0f)
+        / float{EARLY_LINE_LENGTHS.size()} * density_mult;
+    Coeff = CalcDecayCoeff(length, decayTime);
+
 }
 
 /* Update the EAX modulation step and depth. Keep in mind that this kind of
@@ -1551,11 +1556,11 @@ void ReverbPipeline::processEarly(const DelayLineU &main_delay, size_t offset,
 
         /* Apply a delay and bounce to generate secondary reflections. */
         early_delay.writeReflected(offset, tempSamples, todo);
+        const auto feedb_coeff = mEarly.Coeff;
         for(size_t j{0_uz};j < NUM_LINES;j++)
         {
             const auto input = early_delay.get(j);
             auto feedb_tap = size_t{offset - mEarly.Offset[j]};
-            const auto feedb_coeff = float{mEarly.Coeff[j]};
             auto out = outSamples[j].begin() + base;
             auto tmp = tempSamples[j].begin();