lavf: introduce AVFMT_TS_NEGATIVE

Most formats do not support negative timestamps, shift them to avoid
unexpected behaviour and a number of bad crashes.

CC:libav-stable@libav.org

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
x86: Rename dsputil_rnd_template.c to rnd_template.c

This makes it less confusing when this template is shared both by
dsputil and by hpeldsp.

Signed-off-by: Martin Storsjö <martin@martin.st>