Fix a really dumb bug in the caching video provider that caused crashes when seeking in the video.

Originally committed to SVN as r2704.

aegisub/video_provider_cache.cpp

@@ -129,8 +129,8 @@ void VideoProviderCache::Cache(int n,const AegiVideoFrame frame) {
 	}
 
 	// Cache
-	cache.front().n = n;
-	cache.front().frame.CopyFrom(frame);
+	cache.back().n = n;
+	cache.back().frame.CopyFrom(frame);
 }