we can scroll

we can scroll
This adds a scrollbar to allow moving the viewport around, and the
scrollbar does stay in the right place as one changes the zoom level.

However there's a bit of weirdness with it. Currently the minimum zoom
level is "2" as, 1 causes the display to be at half size. (probalby
due to the the curOrtho left and right having the scale added and
subtracted from them, respectively).

I also think the scrollbar's going to be very grumpy for sequences larger
than 100k. as its width isn't scaled, and that's going to make a
very hard to control scrollbar.