matroskademux, avidemux: implement accurate keyframe snapping seeks

Submitted by Vincent Penquerc'h @vincent

Link to original bug (#676630)

Description

When the ACURATE flag is set for snapping keyframe seeks, we seek to the
previous known keyframe and scan forward till the required keyframe is
found. For backward snapping, this will require an extra seek.

Note that this will be unneeded if all keyframes are indexed. I have no
idea whether Matroska requires this or not.