Fix optional arguments of map_create_pixelart tool

According to cppcheck's `knownConditionTrueFalse` error:

```
src\tools\map_create_pixelart.cpp:58:24: style: Condition 'argc>=10' is always true [knownConditionTrueFalse]
 aArtOptions[0] = argc >= 10 ? str_toint(argv[10]) : true; //optimize
                       ^
src\tools\map_create_pixelart.cpp:34:10: note: Assuming that condition 'argc<11' is not redundant
 if(argc < 11 || argc > 12)
         ^
src\tools\map_create_pixelart.cpp:58:24: note: Condition 'argc>=10' is always true
 aArtOptions[0] = argc >= 10 ? str_toint(argv[10]) : true; //optimize
                       ^
src\tools\map_create_pixelart.cpp:59:24: style: Condition 'argc>=11' is always true [knownConditionTrueFalse]
 aArtOptions[1] = argc >= 11 ? str_toint(argv[11]) : false; //centralize
                       ^
src\tools\map_create_pixelart.cpp:34:10: note: Assuming condition 'argc<11' is false
 if(argc < 11 || argc > 12)
         ^
src\tools\map_create_pixelart.cpp:59:24: note: Condition 'argc>=11' is always true
 aArtOptions[1] = argc >= 11 ? str_toint(argv[11]) : false; //centralize
                       ^
```

src/tools/map_create_pixelart.cpp

@@ -31,7 +31,7 @@ int main(int argc, const char **argv)
 	CCmdlineFix CmdlineFix(&argc, &argv);
 	log_set_global_logger_default();
 
-	if(argc < 11 || argc > 12)
+	if(argc < 9 || argc > 12)
 	{
 		dbg_msg("map_create_pixelart", "Invalid arguments");
 		dbg_msg("map_create_pixelart", "Usage: %s <image.png> <img_pixelsize> <input_map> <layergroup_id> <layer_id> <pos_x> <pos_y> <quad_pixelsize> <output_map> [optimize=0|1] [centralize=0|1]", argv[0]);