SDL: Fix display artefacts with transparent OSD message

If we do not update the area below the message, it is just blitted on top
of itself again and again and gets progressively less transparent. It also
causes artefacts when the mouse pass below the OSD message.

backends/graphics/surfacesdl/surfacesdl-graphics.cpp

@@ -2366,6 +2366,7 @@ void SurfaceSdlGraphicsManager::removeOSDMessage() {
 	// Remove the previous message
 	if (_osdMessageSurface) {
 		SDL_FreeSurface(_osdMessageSurface);
+		_forceRedraw = true;
 	}
 
 	_osdMessageSurface = NULL;
@@ -2387,16 +2388,15 @@ void SurfaceSdlGraphicsManager::updateOSD() {
 				_osdMessageAlpha = startAlpha + diff * (SDL_ALPHA_TRANSPARENT - startAlpha) / kOSDFadeOutDuration;
 			}
 			SDL_SetAlpha(_osdMessageSurface, SDL_RLEACCEL | SDL_SRCALPHA, _osdMessageAlpha);
-			_forceRedraw = true;
 		}
 
 		if (_osdMessageAlpha == SDL_ALPHA_TRANSPARENT) {
 			removeOSDMessage();
 		}
 	}
 
-	if (_osdIconSurface) {
-		// Redraw the area below the icon for the transparent blit to give correct results.
+	if (_osdIconSurface || _osdMessageSurface) {
+		// Redraw the area below the icon and message for the transparent blit to give correct results.
 		_forceRedraw = true;
 	}
 }