AP_VideoTX: Add pull down VTX option

libraries/AP_VideoTX/AP_VideoTX.cpp

@@ -61,7 +61,7 @@ const AP_Param::GroupInfo AP_VideoTX::var_info[] = {
     // @DisplayName: Video Transmitter Options
     // @Description: Video Transmitter Options. Pitmode puts the VTX in a low power state. Unlocked enables certain restricted frequencies and power levels. Do not enable the Unlocked option unless you have appropriate permissions in your jurisdiction to transmit at high power levels.
     // @User: Advanced
-    // @Bitmask: 0:Pitmode,1:Pitmode until armed,2:Pitmode when disarmed,3:Unlocked
+    // @Bitmask: 0:Pitmode,1:Pitmode until armed,2:Pitmode when disarmed,3:Unlocked,4:Add leading zero byte to requests
     AP_GROUPINFO("OPTIONS",  6, AP_VideoTX, _options, 0),
 
     // @Param: MAX_POWER

libraries/AP_VideoTX/AP_VideoTX.h

@@ -45,6 +45,7 @@ public:
         VTX_PITMODE_UNTIL_ARM = (1 << 1),
         VTX_PITMODE_ON_DISARM = (1 << 2),
         VTX_UNLOCKED          = (1 << 3),
+        VTX_PULLDOWN          = (1 << 4),
     };
 
     static const char *band_names[];