Add API to output a single duplicate for the *previous* frame
The use case for this is, when you only know how many duplicate frames there will be after seeing the next non-duplicate frame. This will increase latency a lot and is quite suboptimal.
TH_ENCCTL_SET_DUP_COUNT requires to know the number of duplicate frames of the next frame in advance unfortunately.
This could maybe be implemented by allowing to pass NULL as buffer to th_encode_ycbcr_in().