opus/matroska: Adding support for End Trimming in demuxer/decoder

Implementing support for end trimming Opus in Matroska by making
use of the DiscardPadding value from the container and discarding
the samples accordingly.

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