lsmashinput: Initialize get_buffer() of the decoder configuration handler by defalut get_buffer().

Fix a crash when LW-Libav reader is used and there is extradata change.
vslsmas: Rename 'cache_index' into 'cache'.
lwlibav: Integrate common finder of the first valid frame.
libavsmash: Integrate common finder of the first valid frame.
lwlibav: Don't assume frames which has PTS == DTS as B-picture during PTS generation.

The assumption might have considered all frames as B-picture when there is no B-picture.
lwlibav: Add file offset correction for video seek which is available if no invalid file offset.
resample: Remove unused field.