DiabloUI: Support non-doublebuf HW surfaces

If using a hardware surface without double-buffering, perform
rendering in a single blit to avoid flickering and tearing.

Source/DiabloUI/diabloui.cpp

@@ -623,6 +623,8 @@ void LoadBackgroundArt(const char *pszFile, int frames)
 	fadeValue = 0;
 	BlackPalette();
 	SDL_FillRect(DiabloUiSurface(), NULL, 0x000000);
+	if (DiabloUiSurface() == pal_surface)
+		BltFast(nullptr, nullptr);
 	RenderPresent();
 }
 
@@ -655,6 +657,8 @@ void UiFadeIn()
 		}
 		SetFadeLevel(fadeValue);
 	}
+	if (DiabloUiSurface() == pal_surface)
+		BltFast(nullptr, nullptr);
 	RenderPresent();
 }
 

Source/DiabloUI/diabloui.h

@@ -80,7 +80,18 @@ extern bool (*gfnHeroInfo)(bool (*fninfofunc)(_uiheroinfo *));
 
 inline SDL_Surface *DiabloUiSurface()
 {
-	return GetOutputSurface();
+	auto *output_surface = GetOutputSurface();
+
+#ifdef USE_SDL1
+	// When using a non double-buffered hardware surface, render the UI
+	// to an off-screen surface first to avoid flickering / tearing.
+	if ((output_surface->flags & SDL_HWSURFACE) != 0
+	    && (output_surface->flags & SDL_DOUBLEBUF) == 0) {
+		return pal_surface;
+	}
+#endif
+
+	return output_surface;
 }
 
 void UiDestroy();