tools: display the discrete axis steps too

tools: display the discrete axis steps too

Draw a second smaller scroll bar that moves with every discrete step. For that
to work, we have to accumulate the value from the normal scroll events until
we get the first discrete one, then move up.

The value per discrete event changes depending on the click wheel angle, so we
can't just use discrete on its own if we want the two scroll bars aligned.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>