CVideoHandler.cpp: Clear screen before rendering each frame of the intro

This way, the background is black instead of showing glitchy artifacts when one resizes the window

client/CVideoHandler.cpp

@@ -631,6 +631,7 @@ bool CVideoPlayer::playVideo(int x, int y, bool stopOnKey)
 
 		SDL_Rect rect = CSDL_Ext::toSDL(pos);
 
+		SDL_RenderClear(mainRenderer);
 		SDL_RenderCopy(mainRenderer, texture, nullptr, &rect);
 		SDL_RenderPresent(mainRenderer);