Timetrack fixes and refactoring.

Possibly fixes:

Bug 206 - Time Tracks: Ruler warp goes in wrong direction
Bug 205 - Time Tracks that slow down the audio result in truncated exports

src/TimeTrack.h

@@ -57,6 +57,9 @@ class TimeTrack: public Track {
    // Access the track's speed envelope
 
    Envelope *GetEnvelope() { return mEnvelope; }
+   
+   //Compute the integral warp factor between two non-warped time points
+   double ComputeWarpFactor(double t0, double t1);
 
    // Get/Set the speed-warping range, as percentage of original speed (e.g. 90%-110%)