Fix loading screen glitches

Source/diablo.cpp

@@ -704,8 +704,16 @@ void GameEventHandler(const SDL_Event &event, uint16_t modState)
 			LastMouseButtonAction = MouseActionType::None;
 			sgbMouseDown = CLICK_NONE;
 			ShowProgress(GetCustomEvent(event.type));
+
 			RedrawEverything();
-			DrawAndBlit();
+			if (!HeadlessMode) {
+				while (IsRedrawEverything()) {
+					// In direct rendering mode with double/triple buffering, we need
+					// to prepare all buffers before fading in.
+					DrawAndBlit();
+				}
+			}
+
 			LoadPWaterPalette();
 			if (gbRunGame)
 				PaletteFadeIn(8);

Source/interfac.cpp

@@ -467,6 +467,17 @@ void ShowProgress(interface_mode uMsg)
 	if (!HeadlessMode) {
 		assert(ghMainWnd);
 
+		if (RenderDirectlyToOutputSurface && PalSurface != nullptr) {
+			// Ensure that all back buffers have the full progress bar.
+			const void *initialPixels = PalSurface->pixels;
+			do {
+				DrawCutsceneForeground();
+				if (DiabloUiSurface() == PalSurface)
+					BltFast(nullptr, nullptr);
+				RenderPresent();
+			} while (PalSurface->pixels != initialPixels);
+		}
+
 		PaletteFadeOut(8);
 	}