AP_OSD: take semaphore in OSD thread for scripting override

The scripting bindings took get_semaphore() around every OSD call, but the
OSD thread updated the display without it, so a scripting draw could run
concurrently with the OSD thread and the lock protected nothing.

Hold the semaphore in the OSD thread around the override check and
update_osd(), and set scripting_override under it, so the OSD thread and
scripting draws are mutually exclusive.

libraries/AP_OSD/AP_OSD.cpp

@@ -409,10 +409,11 @@ void AP_OSD::osd_thread()
             update_current_screen();
         }
 #if AP_SCRIPTING_ENABLED
+        // hold the semaphore so scripting cannot draw to the OSD while the
+        // OSD thread is updating it, and vice versa
+        WITH_SEMAPHORE(_sem);
         if (!scripting_override) {
-#endif
             update_osd();
-#if AP_SCRIPTING_ENABLED
         } else {
             override_count++;
             if (override_count > 20) {
@@ -421,6 +422,8 @@ void AP_OSD::osd_thread()
                 override_count = 0;
             }
         }
+#else
+        update_osd();
 #endif // AP_SCRIPTING_ENABLED
     }
 }

libraries/AP_OSD/AP_OSD.h

@@ -659,6 +659,9 @@ public:
     }
 #if AP_SCRIPTING_ENABLED
     AP_OSD_Backend *scripting_get_backend() {
+        // set the override under the semaphore so the OSD thread sees a
+        // consistent value; the scripting binding takes it again for the call
+        WITH_SEMAPHORE(_sem);
         scripting_override = true;
         override_count = 0;
         return _backends[0];