fixed peak waveform issue introduced by the ftruncate preallocation of peakfile....

fixed peak waveform issue introduced by the ftruncate preallocation of peakfile.  also prevented that ftruncate from actually reducing the size of the peakfile in the case of destructive tracks.  changed the sources PeaksReady semantics back to emitting only on peak completion on normal tracks.  truncate peakfiles back to natural size on finish to prevent wasting disk space on shorter takes.

git-svn-id: svn://localhost/ardour2/trunk@1250 d708f5d6-7413-0410-9779-e7cbd77b26cf