Plane: allow landing height adjustment on wp before land (aka approach wp)

ArduPlane/Plane.h

@@ -442,6 +442,9 @@ private:
         // are we in auto and flight mode is approach || pre-flare || final (flare)
         bool land_in_progress:1;
 
+        // are we headed to the land approach waypoint? Works for any nav type
+        bool wp_is_land_approach:1;
+
         // should we fly inverted?
         bool inverted_flight:1;
 

ArduPlane/altitude.cpp

@@ -474,7 +474,8 @@ int32_t Plane::adjusted_relative_altitude_cm(void)
 float Plane::mission_alt_offset(void)
 {
     float ret = g.alt_offset;
-    if (control_mode == AUTO && auto_state.land_in_progress) {
+    if (control_mode == AUTO &&
+            (auto_state.land_in_progress || auto_state.wp_is_land_approach)) {
         // when landing after an aborted landing due to too high glide
         // slope we use an offset from the last landing attempt
         ret += auto_state.land_alt_offset;

ArduPlane/commands_logic.cpp

@@ -40,6 +40,10 @@ bool Plane::start_command(const AP_Mission::Mission_Command& cmd)
         // reset loiter start time. New command is a new loiter
         loiter.start_time_ms = 0;
 
+        AP_Mission::Mission_Command next_nav_cmd;
+        const uint16_t next_index = mission.get_current_nav_index() + 1;
+        auto_state.wp_is_land_approach = mission.get_next_nav_cmd(next_index, next_nav_cmd) && (next_nav_cmd.id == MAV_CMD_NAV_LAND);
+
         gcs_send_text_fmt(MAV_SEVERITY_INFO, "Executing nav command ID #%i",cmd.id);
     } else {
         gcs_send_text_fmt(MAV_SEVERITY_INFO, "Executing command ID #%i",cmd.id);