change sliderValue() so monitor is correctly updated

 when slider is inverted


Error example:
- Have an inverted fader monitoring a channel
- Set the channel from 0 to 255 with a button
- The monitoring fader will not move

This is because the test for changing the fader value is done with sliderValue(), and sliderValue() does not know about inverted faders.
Here we get the correct value in sliderValue().