Fix #286: alf_ctb_flag parsing condition for chroma
According to specification, alf_ctb_flag is parsed for Cb when (slice_alf_chroma_idc = 1 or slice_alf_chroma_idc = 3), and for Cr when (slice_alf_chroma_idc = 2 or slice_alf_chroma_idc = 3).
Edited by Remy Foray