FFmpeg: Improve scrubbing performance

Applying negative offset to seek position before scanning doesnn't have
any effect. This change results in 1.5x faster seeking (random frame,
average value) in sample file with 30 frame GOP length.

If I am not mistaken, B frames can have `pts` that can be less than
`pts` of I frame that must be decoded. Even in this case though, B frame
packet will be stored after that I frame.

In addition, preseek value is de facto hardcoded so seeking would fail
if it could. This can be hard to spot though.

Differential Revision: https://developer.blender.org/D10529