vaapi: mpeg2: fix slice_vertical_position calculation.

VASliceParameterBufferMPEG2.slice_vertical_position shall express
the slice vertical position from the original bitstream. The HW
decoder will correctly decode to the right line computed from the
appropriate top_field_first and is_first_field flags.

This patch aligns with DXVA's definition, which is what most HW and
drivers expect. In particular, Intel PowerVR (Cedarview et al.) and
NVIDIA (through VA-to-VDPAU layer). Since it looks more complex to fix
binary drivers, I aligned the Intel Gen driver (Sandy Bridge et al.)
to this behaviour, while maintaining compatibility with codec layers
not providing this patch yet.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vaapi_mpeg2.c

@@ -132,7 +132,7 @@ static int vaapi_mpeg2_decode_slice(AVCodecContext *avctx, const uint8_t *buffer
         return -1;
     slice_param->macroblock_offset              = macroblock_offset;
     slice_param->slice_horizontal_position      = s->mb_x;
-    slice_param->slice_vertical_position        = s->mb_y;
+    slice_param->slice_vertical_position        = s->mb_y >> (s->picture_structure != PICT_FRAME);
     slice_param->quantiser_scale_code           = quantiser_scale_code;
     slice_param->intra_slice_flag               = intra_slice_flag;
     return 0;