Merge r3302 into 2.1.8 so dummy video works again there too.

Originally committed to SVN as r3303.

aegisub/src/video_provider_dummy.h

@@ -77,6 +77,7 @@ public:
 	bool AreKeyFramesLoaded() { return false; };
 	wxArrayInt GetKeyFrames() { return wxArrayInt(); };
 	bool IsVFR() { return false; };
+	bool NeedsVFRHack() { return true; }
 	FrameRate GetTrueFrameRate() { return FrameRate(); };
 
 	Aegisub::String GetDecoderName();