lavf: inspect more frames for fps when container time base is coarse

As per issue2629, most 23.976fps matroska H.264 files are incorrectly
detected as 24fps, as the matroska timestamps usually have only
millisecond precision.

Fix that by doubling the amount of timestamps inspected for frame rate
for streams that have coarse time base. This also fixes 29.970 detection
in matroska.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 78431098)

Tested with mplayer based on this report
http://thread.gmane.org/gmane.comp.video.mplayer.user/66043/focus=66063



Signed-off-by: Reinhard Tartler <siretart@tauware.de>