Convert Welte red roll acceleration emulation method from displacement- to time-based

[broadwell]

This could be implemented via a relatively straightforward modification of the emulation code, which presently increases the tempo by .22% at 12-inch intervals along the roll (converted to MIDI tick values which are pegged at 1 tick = 1 pixel; 12 inches at 300 ppi = 3600 ticks). The updated code would take into account that the amount of time the roll takes to traverse the second foot of the roll is shorter than for the first foot as the uptake speed increases; the result would be that the .22% increases in tempo would occur at increasing tick intervals along the length of the roll, though the difference in the acceleration profile from the displacement-based approach across standard-length rolls would be quite small.