[Encoder] for reference selection under screen

1, ref_list_mgr_svc.cpp, Ln314: fix a wrong DeleteLTRFromLongList which should not be used under screen strategy
2, ref_list_mgr_svc.cpp, Ln910: remove the frame which is furthest in distance to the current frame in ref list
3, wels_preprocess.cpp: use scene LTR ref list when the current frame is scene LTR
-, ref_list_mgr_svc.cpp, Ln811: add DEBUG trace
reviewed at: https://rbcommons.com/s/OpenH264/r/870/