ffmpeg: use timestamp from before and after the decoder to detect backward moving dts for discontinuity detection

This should make discontinuity detection more robust and detect
more cases. Timestamps after the decoder can be delayed due to threads,
timestamps before can be missing

no testcase available/known

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
configure: support raising major version in soname

this allows seperate installation of shared libs that should not conflict with
whatever is already installed.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>