default the output file name

src/fart/Engine.cpp

@@ -7,5 +7,19 @@ Engine::Engine(const RenderParams & params)
 
 int Engine::run(string_view input_file, opt<string_view> output_file)
 {
+    string output_path;
+    if (output_file)
+    {
+        output_path = *output_file;
+    }
+    else
+    {
+        string_view base = input_file;
+        if (base.ends_with(".fart"))
+        {
+            base.remove_suffix(5u);
+        }
+        output_path = string(base) + ".png";
+    }
     return 0;
 }