fix major/minor ticks on min:sec ruler

All ticks from 0:00.000 to 0:00.999... were major, and everything after
was minor, instead of the correct distribution.
fix rounding error in the min:sec ruler

When zoomed in to the millisecond level, the framerate was being divided
by 1000 as an integer to generate a ruler tick interval, which doesn't
work so well at things like 44100 or 88200.  Instead, just count this
value in milliseconds, dividing by 1000 when we are done.

This was purely a display issue - the grid was in the correct place.