Actually flush to disk continuously while MIDI loop recording.

Actually flush to disk continuously while MIDI loop recording.

Previously, this calculation was based on the current transport frame and last
time, both absolute, with the effect that loop recording would flush the first
time and never again.

The fix is to keep track of the actual duration of the capture, ignoring the
transport position, and flush as that reaches the threshold.

git-svn-id: svn://localhost/ardour2/branches/3.0@13938 d708f5d6-7413-0410-9779-e7cbd77b26cf