Limit number of video frames to look for VFR.
Limit number of video frames to look for VFR.

This reworks find_first_pts() after the initial implementation of VFR
detection and subsequently discovering a regression by naively limiting
the range of this search. Now, there is a separate countdown for VFR
that can be used to cancel the loop if first_pts has been determined.

mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt