AI braking: keep braking for a lower speed inside the proximity range

Inside the proximity range only a stop had a final approach of its own;
for a lower speed ahead that branch set nothing, so AccDesired stayed at
the preferred acceleration. With the range at 50 m, which this PR uses so
that trains stop 8-15 m short of a signal, the AI stopped braking within
the last 50 m before a speed restriction and even took power: EMUs reached
a 20 km/h restriction at 25 km/h. On master the range is 15-40 m, which
kept this mostly out of sight.

A lower speed ahead now keeps the braking curve right up to where it
begins. Stops are unchanged.

Measured in real time on current master (with #199), with a 20 km/h
restriction one coach long, two runs of 32 consists each: over 22 km/h in
the restriction on the flat 9 on master, 15 before this change, 1 after
(a 36WEa at 22.1 km/h); on the 20 per mille descent 15, 0 and 0. Stops
8-15 m short of the signal stay as they were (flat 57 and 55 of 64,
descent 44 and 42), no overruns.

vehicle/Driver.cpp

@@ -7518,7 +7518,10 @@ TController::adjust_desired_speed_for_target_speed( double const Range ) {
             }
             else if (ActualProximityDist > fMinProximityDist) {
                 // jedzie szybciej, niż trzeba na końcu ActualProximityDist, ale jeszcze jest daleko
-				if (ActualProximityDist < fMaxProximityDist) {
+				// only a stop has a final approach of its own inside the proximity range; a lower speed ahead
+				// keeps the braking curve below right up to where it begins, or the train would stop braking
+				// (and even take power) that far out
+				if( ( VelNext == 0.0 ) && ( ActualProximityDist < fMaxProximityDist ) ) {
                     // jak minął już maksymalny dystans po prostu hamuj (niski stopień)
                     // ma stanąć, a jest w drodze hamowania albo ma jechać
 /*