Fill in new VASliceParameterBufferMPEG2.slice_horizontal_position field.

This is needed for the Intel G45 driver.

Originally committed as revision 19876 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vaapi_mpeg2.c

@@ -131,6 +131,7 @@ static int vaapi_mpeg2_decode_slice(AVCodecContext *avctx, const uint8_t *buffer
     if (!slice_param)
         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->quantiser_scale_code           = quantiser_scale_code;
     slice_param->intra_slice_flag               = intra_slice_flag;