use absolute values again

- calculate differences of incomming wheel signals
- sum them for outgoing wheel signal
add brightness offset for night view

- start with 64 as default
- daylight ok so far
- resistor changed to 18k instead of 38k as in the schematics
- light depending resistor varies between 7k and 180k
change SPI prescaling factor to 4

- faster SPI throughput
correct rollover logic once more for wheel ticks

- now use the correct direction for the delta calculation
  when rolling over