slider issue 2.6.1 r8397

[Jan Gruuthuse]

Slider issues:
- not moving from start position on some avi
- only moving to keyframes on some mp4
- seems to be working correctly on mpeg-ts

Kernel: Linux 3.2.0-35-generic (x86_64)
Distribution: Linux Ubuntu 12.04.1 LTS (Precise Pangolin)