AP_TECS: Added method to handle home altitude change

libraries/AP_TECS/AP_TECS.cpp

@@ -1501,3 +1501,30 @@ void AP_TECS::_update_pitch_limits(const int32_t ptchMinCO_cd) {
     // don't allow max pitch to go below min pitch
     _PITCHmaxf = MAX(_PITCHmaxf, _PITCHminf);
 }
+
+void AP_TECS::offset_altitude(const float alt_offset)
+{
+    // Convention: When alt_offset is positive it means that the altitude of
+    // home has increased. Thus, the relative altitude of the vehicle has
+    // decreased.
+    //
+    // Assumption: This method is called more often and before
+    // `update_pitch_throttle()`. This is necessary to ensure that new height
+    // demands which incorporate the home change are compatible with the
+    // (now updated) internal height state.
+
+    _flare_hgt_dem_ideal    -= alt_offset;
+    _flare_hgt_dem_adj      -= alt_offset;
+    _hgt_at_start_of_flare  -= alt_offset;
+    _hgt_dem_in_prev        -= alt_offset;
+    _hgt_dem_lpf            -= alt_offset;
+    _hgt_dem_rate_ltd       -= alt_offset;
+    _hgt_dem_prev           -= alt_offset;
+    _height_filter.height   -= alt_offset;
+
+    // The following variables are updated anew in every call of
+    // `update_pitch_throttle()`. There's no need to update those.
+    // _hgt_dem
+    // _hgt_dem_in_raw
+    // _hgt_dem_in
+}

libraries/AP_TECS/AP_TECS.h

@@ -154,6 +154,9 @@ public:
         _need_reset = true;
     }
 
+    // Apply an altitude offset, to compensate for changes in home alt.
+    void offset_altitude(const float alt_offset);
+
     // this supports the TECS_* user settable parameters
     static const struct AP_Param::GroupInfo var_info[];