hwaccel: mpeg2: decode first field, if requested.

If user opted to present fields as they come, then the first field
picture needs to be submitted to the HW for decoding. In particular,
this fixes MPEG-2 decoding of interlaced streams.

Tested on Intel Cedar Trail, Sandy Bridge and Ivy Bridge platforms.
Someone reported on the ffmpeg-devel@ list this also works on DXVA
(Windows) and other Linux platforms (NVIDIA, through the VA wrapper).

This also means a similar patch to non-hwaccel VDPAU may be necessary.

Note: I believe the SLICE_FLAG_ALLOW_FIELD is useless since the first
field shall always be submitted to the HW anyway. Nobody uses HW accels
(dxva, vaapi, vdpau, etc.) without that flag though.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/mpeg12.c

@@ -1590,6 +1590,12 @@ static int mpeg_field_start(MpegEncContext *s, const uint8_t *buf, int buf_size)
             return -1;
         }
 
+        if (s->avctx->hwaccel &&
+            (s->avctx->slice_flags & SLICE_FLAG_ALLOW_FIELD)) {
+            if (s->avctx->hwaccel->end_frame(s->avctx) < 0)
+                av_log(avctx, AV_LOG_ERROR, "hardware accelerator failed to decode first field\n");
+        }
+
         for (i = 0; i < 4; i++) {
             s->current_picture.f.data[i] = s->current_picture_ptr->f.data[i];
             if (s->picture_structure == PICT_BOTTOM_FIELD) {