Restore change of Waveform dB scale with change of magnification...

... This is done with Shift+Ctrl+wheel and pointer in the vertical ruler, and
the pointer y coordinate near the bottom of the dB scale.

If y coordinate is far from there, then Shift+Ctrl+wheel changes dB level
without change of magnification (as before this commit).

src/TrackArtist.h

@@ -142,6 +142,7 @@ class AUDACITY_DLL_API TrackArtist {
    void DrawWaveformBackground(wxDC & dc, int leftOffset, const wxRect &rect,
                                const double env[],
                                float zoomMin, float zoomMax,
+                               int zeroLevelYCoordinate,
                                bool dB, float dBRange,
                                double t0, double t1, const ZoomInfo &zoomInfo,
                                bool drawEnvelope, bool bIsSyncLockSelected);