avfilter/framesync: fix forward EOF pts

avfilter/framesync: fix forward EOF pts

Note1: when the EOF pts is not accurate enough, the last frame
can be dropped by vf_fps with default rounding.

Note2: vf_scale use framesync since e82a3997cdd6c0894869b33ba42430ac3,
so this is a very commonplace scenario.

For example:
./ffprobe -f lavfi testsrc=d=1,scale,fps -of flat \
  -count_frames -show_entries stream=nb_read_frames

Before:
streams.stream.0.nb_read_frames="24"

After:
streams.stream.0.nb_read_frames="25"

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit de976eaf30df33e86c58c8c9af9905c1d8441934)
Signed-off-by: Anton Khirnov <anton@khirnov.net>
Gbp-Pq: Name 0012-avfilter-framesync-fix-forward-EOF-pts.patch