Improve incline coding visuals and use different colors for going uphill
Improve incline coding visuals and use different colors for going uphill

Currently it is a bit difficult to guess the slope of the route for
planning purposes, since even for routes in very hilly terrain the
colors hardly change. Only extremely steep hills are indicated,
and there is no visual difference between going uphill or downhill.

By cutting off earlier, more of the route will show meaningful
differences in color. Note that BRouter's gradients are already
averaged compared to the maximum gradients shown on road signs, so 15%
should be a good compromise (anything steeper is difficult to ride on
for longer periods anyway, and rightly deserves to be colored with an
alarming red).

By using different colors for the min and max parameters, uphill and
downhill sections should now be easy to distinguish.

By introducing two more color stops, the gradient becomes much smoother,
with flat sections featuring a distinctive green, where they were drawn
in a muddy dark green before.

Note that this approach had been implemented like this in QLandkarte GT
for several years now, with great success.

nrenner/brouter-web,nrenner/brouter-web,nrenner/brouter-web