lavc: Adding seek_preroll to AVCodecContext

seek_preroll field is added to the AVCodecContext struct. It indicates
the number of samples to be discarded whenever there is a discontinuity.
The minor version is bumped accordingly.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>