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>
Merge commit 'c34a96a5ddfa390ce2a352eca79190766c9056d4'

* commit 'c34a96a5ddfa390ce2a352eca79190766c9056d4':
  dxa: fix decoding of first I-frame by separating I/P-frame decoding

Conflicts:
	libavcodec/dxa.c

See: 186e47ef6d7d90bfd8b16e77cfa5e7aeb2a497c0
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  movenc: Indicate that negative timestamps are supported

Merged-by: Michael Niedermayer <michaelni@gmx.at>
fate: add ffv1.0 test

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