…line mode

The two pass approach didn't help, and made lines which were drawn straight
actually appear curved when you released the mouse.  The problem actually
appeared to be coming from needing to handle points a little beyond the end of
the "toAlign" array, so I had to make the interpolate function extrapolate off
the right end of that line.  Also fixed a problem with inserting lots of
"minimum possible value" events if the user released the mouse below the bottom
edge of the window.

…es correctly

In particular, I still need to figure out what the righthand bound of the range
should be for removing the old events.  maxTick isn't correct, even if you
recompute it as xPosToTock(toAlign.last().first) each time you go around the
loop, because the end point depends on the tempo events you're going to insert
not the ones which are already there.  I may need to intersperse the remove and
add rather than having them in separate loops.

What you draw is evaluated in the context of what's already there, so that
means that a tempo line which you draw straight will end up curved once you
release the mouse.  That's unintuitive, but I'm starting to think it's not
actually a bug, just an accurate reflection of what the straight line really
means.