Fix garbage frame scan-out during WAD loading

Implement stable black frame solution that presents known frames
before blocking WAD load operations to prevent garbage scan-out
on both TV and GamePad displays.

Changes:
- Add I_PresentBlackFrame() helper function in i_video.c
- Call I_PresentBlackFrame() before IWAD loading (D_AddFile)
- Call I_PresentBlackFrame() before PWAD loading (W_ParseCommandLine)
- Present two frames to ensure both TV and GamePad buffers are valid
- Only affects Wii U builds via __WIIU__ preprocessor guards

This solution addresses the root cause: during WAD loading the main
thread blocks, no SDL_RenderPresent() calls occur, and GX2's scan-outs
begin scanning uninitialised back buffers. By presenting stable black
frames before the blocking operations, both displays show clean content
during loading transitions.

Author: strejf79

src/doom/d_main.c

@@ -1688,6 +1688,10 @@ void D_DoomMain (void)
     modifiedgame = false;
 
     DEH_printf("W_Init: Init WADfiles.\n");
+#ifdef __WIIU__
+    // Present stable black frame before blocking WAD load to prevent garbage scan-out
+    I_PresentBlackFrame();
+#endif // __WIIU__
     D_AddFile(iwadfile);
     numiwadlumps = numlumps;
 
@@ -1834,6 +1838,10 @@ void D_DoomMain (void)
     DEH_ParseCommandLine();
 
     // Load PWAD files.
+#ifdef __WIIU__
+    // Present stable black frame before blocking PWAD load to prevent garbage scan-out
+    I_PresentBlackFrame();
+#endif // __WIIU__
     modifiedgame = W_ParseCommandLine();
 
     //!

src/i_video.c

@@ -60,6 +60,16 @@ int WIDESCREENDELTA; // [crispy] horizontal widescreen offset
 static SDL_Window *screen;
 static SDL_Renderer *renderer;
 
+// Helper function to present a stable black frame during blocking operations
+void I_PresentBlackFrame(void) {
+    if (renderer != NULL) {
+        SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
+        SDL_RenderClear(renderer);
+        SDL_RenderPresent(renderer);
+        SDL_RenderPresent(renderer); // second call ensures both TV and GamePad buffers are valid
+    }
+}
+
 // Window title
 
 static const char *window_title = "";

src/i_video.h

@@ -51,6 +51,7 @@ typedef boolean (*grabmouse_callback_t)(void);
 // determines the hardware configuration
 // and sets up the video mode
 void I_InitGraphics (void);
+void I_PresentBlackFrame(void);
 
 void I_GraphicsCheckCommandLine(void);