loop recording fixes

I've just pushed a bunch of fixes for looped recording to ardour's git
repository.

I'd appreciate it if people would give them a try. Looped recording
has been basically broken in various ways for a long time, particular
when using non-seamless looping which I suspect has *never* worked.

Feedback is welcome. A hint (from Ben Loftis @ Harrison) that makes
loop recording more useful: set up the punch range inside the loop
range, starting later than the loop. This gives you some "pre-roll" to
allow more natural and precise punches.

I'm not 100% interested in radical ways to improve looped recording -
more just looking for confirmation that things work now - but I'll
listen if people have such ideas.

thanks,
--p