Copter: change altitude fence to work on home altitudes instead of origin altitudes

ArduCopter/control_rtl.pde

@@ -401,7 +401,7 @@ static float get_RTL_alt()
 #if AC_FENCE == ENABLED
     // ensure not above fence altitude if alt fence is enabled
     if ((fence.get_enabled_fences() & AC_FENCE_TYPE_ALT_MAX) != 0) {
-        rtl_alt = min(rtl_alt, fence.get_safe_alt()*100.0f);
+        rtl_alt = min(rtl_alt, pv_alt_above_origin(fence.get_safe_alt()*100.0f));
     }
 #endif
 

ArduCopter/fence.pde

@@ -20,7 +20,7 @@ void fence_check()
     fence.set_home_distance(home_distance*0.01f);
 
     // check for a breach
-    new_breaches = fence.check_fence();
+    new_breaches = fence.check_fence(pv_alt_above_home(inertial_nav.get_altitude()));
 
     // if there is a new breach take action
     if( new_breaches != AC_FENCE_TYPE_NONE ) {