Change the drawing of wave track envelope at high zoom...

... The function defined by the envelope is evaluated exactly at the times of
samples, but not between samples -- instead there is a simple interpolation.

Therefore the curve might not go through the control points when they are not at
sample times.

The exact value of the function defined by the envelope has no influence on
rendering of the sound in between samples.  So this can make it clear that
the middle point has no influence at all in case three points are very close.

Drawing of other envelopes (Time track, Equalization curves) is not changed.

src/Envelope.cpp

@@ -1306,10 +1306,45 @@ void Envelope::GetValuesRelative(double *buffer, int bufferLen,
 }
 
 void Envelope::GetValues
-   (double *buffer, int bufferLen, int leftOffset, const ZoomInfo &zoomInfo) const
+   ( double alignedTime, double sampleDur,
+     double *buffer, int bufferLen, int leftOffset,
+     const ZoomInfo &zoomInfo )
+   const
 {
-   for (int xx = 0; xx < bufferLen; ++xx)
-      buffer[xx] = GetValue(zoomInfo.PositionToTime(xx, -leftOffset));
+   // Getting many envelope values, corresponding to pixel columns, which may
+   // not be uniformly spaced in time when there is a fisheye.
+
+   double prevDiscreteTime, prevSampleVal, nextSampleVal;
+   for ( int xx = 0; xx < bufferLen; ++xx ) {
+      auto time = zoomInfo.PositionToTime( xx, -leftOffset );
+      if ( sampleDur <= 0 )
+         // Sample interval not defined (as for time track)
+         buffer[xx] = GetValue( time );
+      else {
+         // The level of zoom-in may resolve individual samples.
+         // If so, then instead of evaluating the envelope directly,
+         // we draw a piecewise curve with knees at each sample time.
+         // This actually makes clearer what happens as you drag envelope
+         // points and make discontinuities.
+         auto leftDiscreteTime = alignedTime +
+            sampleDur * floor( ( time - alignedTime ) / sampleDur );
+         if ( xx == 0 || leftDiscreteTime != prevDiscreteTime ) {
+            prevDiscreteTime = leftDiscreteTime;
+            prevSampleVal =
+               GetValue( prevDiscreteTime, sampleDur );
+            nextSampleVal =
+               GetValue( prevDiscreteTime + sampleDur, sampleDur );
+         }
+         auto ratio = ( time - leftDiscreteTime ) / sampleDur;
+         if ( GetExponential() )
+            buffer[ xx ] = exp(
+               ( 1.0 - ratio ) * log( prevSampleVal )
+                  + ratio * log( nextSampleVal ) );
+         else
+            buffer[ xx ] =
+               ( 1.0 - ratio ) * prevSampleVal + ratio * nextSampleVal;
+      }
+   }
 }
 
 // relative time