Plane: fixed combination of passby and acceptance dist WP

when a user sets a passby distance we should calculate the turn point
based on the extrapolated distance, not the original waypoint

also simplify the passby logic using offset_bearing()

ArduPlane/commands_logic.cpp

@@ -615,13 +615,11 @@ bool Plane::verify_nav_wp(const AP_Mission::Mission_Command& cmd)
     uint8_t cmd_acceptance_distance = LOWBYTE(cmd.p1); // radius in meters to accept reaching the wp
 
     if (cmd_passby > 0) {
-        float dist = prev_WP_loc.get_distance(flex_next_WP_loc);
+        const float dist = prev_WP_loc.get_distance(flex_next_WP_loc);
+        const float bearing_deg = degrees(prev_WP_loc.get_bearing(flex_next_WP_loc));
 
-        if (!is_zero(dist)) {
-            float factor = (dist + cmd_passby) / dist;
-
-            flex_next_WP_loc.lat = flex_next_WP_loc.lat + (flex_next_WP_loc.lat - prev_WP_loc.lat) * (factor - 1.0f);
-            flex_next_WP_loc.lng = flex_next_WP_loc.lng + Location::diff_longitude(flex_next_WP_loc.lng,prev_WP_loc.lng) * (factor - 1.0f);
+        if (is_positive(dist)) {
+            flex_next_WP_loc.offset_bearing(bearing_deg, cmd_passby);
         }
     }
 
@@ -651,8 +649,8 @@ bool Plane::verify_nav_wp(const AP_Mission::Mission_Command& cmd)
     } else if (cmd_passby == 0) {
         acceptance_distance_m = nav_controller->turn_distance(get_wp_radius(), auto_state.next_turn_angle);
     }
-
-    if (auto_state.wp_distance <= acceptance_distance_m) {
+    const float wp_dist = current_loc.get_distance(flex_next_WP_loc);
+    if (wp_dist <= acceptance_distance_m) {
         gcs().send_text(MAV_SEVERITY_INFO, "Reached waypoint #%i dist %um",
                           (unsigned)mission.get_current_nav_cmd().index,
                           (unsigned)current_loc.get_distance(flex_next_WP_loc));