lwinput: VFR->CFR conversion for Libav+L-SMASH reader.
lwmuxer: Forbid exporting when VFR->CFR conversion is enabled.

This plugin does not honor this conversion.
libavsmash_video: Fix data type of min_cts.
libavsmash_input: Fix wrong check of return value of au_setup_video_rendering().
lwinput: VFR->CFR conversion for LW-Libav reader.
lwinput: Don't skip writing frame data into AviUtl's frame buffer at the first frame.
lwindex: Try to compute video stream duration if not obtained yet.
lwinput: Set proper last_frame_number at the initial setting for LW-Libav reader.
lwlibav_video: VFR->CFR conversion for invisible frames.
lwlibav_source: Intialize VFR->CFR conversion stuff.
lwinput: Fix seek on VFR->CFR conversion when A/V sync is disabled for Libav+L-SMASH reader.
lsmashsource: Support VFR->CFR conversion.