change semantics of Region::set_length() to preserve current position

Before the nutempo changes, a call to Region::set_length() only set the length
of the region.  Now that Region::_length is a duple of (duration,position),
maintain thesame semantics so that the call only affects the duration part of
the duple.  Theoretically we might at some point want/need
Region::set_length_and_position() but that is not the case at this point,

libs/ardour/region.cc

@@ -491,8 +491,12 @@ Region::set_length (timecnt_t const & len)
 void
 Region::set_length_internal (timecnt_t const & len)
 {
-	_last_length = _length;
-	_length = len;
+	timecnt_t l (len);
+
+	l.set_position (position());
+
+	_last_length = l;
+	_length = l;
 }
 
 void