Monitor: Clip zone polygon to image size when dumping

The polygon vertex coordinated of extra_zone are directly parsed from user input.
Clip the resulting polygon to the image size so we don't try to draw outside of the image.
This is already done for zone polygons stored and loaded from the DB.

src/zm_monitor.cpp

@@ -1481,6 +1481,10 @@ void Monitor::DumpZoneImage(const char *zone_string) {
     zone_image->Colourise(ZM_COLOUR_RGB24, ZM_SUBPIX_ORDER_RGB);
   }
 
+  extra_zone = extra_zone.GetClipped(Box({0, 0},
+                                         {static_cast<int32>(zone_image->Width()),
+                                          static_cast<int32>(zone_image->Height())}));
+
   for (const Zone &zone : zones) {
     if (exclude_id && (!extra_colour || !extra_zone.GetVertices().empty()) && zone.Id() == exclude_id) {
       continue;