Bug1754: Telephone and AM Radio curves should not flatten...

... Problem was a wrongly implemented comparator for wxArray Sort().

The array of points was not really sorted, and results differed with repeated
calls to Sort().

Unlike with std::sort, these comparators are supposed to be trichotomous.

Problem dates to commit 7d5e54e364

See also commit 4a500c77dd

src/effects/Equalization.h

@@ -265,7 +265,12 @@ private:
 
    static int wxCMPFUNC_CONV SortCurvePoints (EQPoint **p0, EQPoint **p1)
    {
-      return (*p0)->Freq > (*p1)->Freq;
+      auto diff = (*p0)->Freq - (*p1)->Freq;
+      if (diff < 0)
+         return -1;
+      if (diff > 0)
+         return 1;
+      return 0;
    }
 
 #ifdef EXPERIMENTAL_EQ_SSE_THREADED