remove use of designated initializers

Added in #2229, this is a C++20 feature. For now we're on C++17 (by way of Node.js v18).

Author: zbjornson

src/Image.cc

@@ -1178,10 +1178,10 @@ Image::renderSVGToSurface() {
   }
 
   RsvgRectangle viewport = {
-    .x = 0,
-    .y = 0,
-    .width = static_cast<double>(width),
-    .height = static_cast<double>(height),
+    0, // x
+    0, // y
+    static_cast<double>(width),
+    static_cast<double>(height)
   };
   gboolean render_ok = rsvg_handle_render_document(_rsvg, cr, &viewport, nullptr);
   if (!render_ok) {