May always scroll before zero during transport when head is pinned...

... regardless of preferences, but if the Tracks preference is not checked,
don't stay scrolled left of zero after stopping.

src/Project.h

@@ -407,6 +407,7 @@ class AUDACITY_DLL_API AudacityProject final : public wxFrame,
 
    void SafeDisplayStatusMessage(const wxChar *msg);
 
+   bool MayScrollBeyondZero() const;
    double ScrollingLowerBoundTime() const;
    // How many pixels are covered by the period from lowermost scrollable time, to the given time:
    // PRL: Bug1197: we seem to need to compute all in double, to avoid differing results on Mac