emu_window: Ensure WindowConfig members are always initialized

Previously we weren't always initializing all members of the struct.
Prevents potentially wonky behavior from occurring.

src/core/frontend/emu_window.h

@@ -32,9 +32,9 @@ class EmuWindow {
 public:
     /// Data structure to store emuwindow configuration
     struct WindowConfig {
-        bool fullscreen;
-        int res_width;
-        int res_height;
+        bool fullscreen = false;
+        int res_width = 0;
+        int res_height = 0;
         std::pair<unsigned, unsigned> min_client_area_size;
     };