Fixed crash when starting up and no proper application config was available.

src/main.cc

@@ -117,6 +117,8 @@ struct Viewport {
   sg_image depth_image = {};
   int size[2] = {400, 400};
 
+  Viewport() { Resize(size[0], size[1]); }
+
   void Resize(int width, int height) {
     width = width < 1 ? 1 : width;
     height = height < 1 ? 1 : height;