Use WaveTrackCache for spectrograms

Zooming-in of spectrograms (or other view changes that invalidate the
whole pixel cache of the WaveClip) used to do at least one opening and
closing of a block file for each column of pixels.  With this change,
open each block file not more than once for each repopulation of the
cache.

Improved speed may be more noticeable on less powerful computers, or when
the audio file is in a slower storage device.

src/TrackArtist.h

@@ -29,6 +29,7 @@ class wxHashTable;
 
 class Track;
 class WaveTrack;
+class WaveTrackCache;
 class WaveClip;
 class NoteTrack;
 class LabelTrack;
@@ -141,7 +142,7 @@ class AUDACITY_DLL_API TrackArtist {
                          bool drawEnvelope, bool drawSamples, bool drawSliders,
                          bool dB, bool muted);
 
-   void DrawClipSpectrum(WaveTrack *track, WaveClip *clip,
+   void DrawClipSpectrum(WaveTrackCache &cache, WaveClip *clip,
                          wxDC & dc, const wxRect & r, const ViewInfo *viewInfo,
                          bool autocorrelation, bool logF);