altivec: perform an explicit unaligned load

Implicit vector loads on POWER7 hardware can use the VSX
instruction set instead of classic Altivec/VMX. Let's force
a VMX load in this case.

Signed-off-by: Martin Storsjö <martin@martin.st>
h261: check the mtype index

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
dxa: Make sure the reference frame exists

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
dxa: fix decoding of first I-frame by separating I/P-frame decoding

5ef7c84 broke decoding for the first keyframe due to an unnecessary
check for a reference frame.

CC: libav-stable@libav.org
movenc: Indicate that negative timestamps are supported

The mov/mp4 muxer has support for handling negative timestamps
via edit lists (which customarily is used for handling the 1-frame
delay due to B-frames as well).

Using the muxer's native way of handling it is better than using
the generic offsetting. The generic offsetting is a bit too
crude when e.g. the timebase of one track is 1/fps, where the
edit lists can handle it accurately.

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