fix reference picture lists checking for multilayer scalable

forcing the short-term ref not being a long-term causes picture not being found, and reissue a RPL in the slice header (causing extra syntax). This fix sets the conditions coherent with "applyReferencePictureListBasedMarking" function, which is called after in the encoder. This is needed because in multilayer, the layer 1 sets the inter layer ref picture of layer 0 as long-term, but the layer 0 picture is a short term ref for subsequent pictures of layer 0. By the way, what is the point of setting ILRP as "long-term" reference pictures (clause 8.3.3 Decoding process for reference picture marking) ?