Used the AVCodecContext given to AVCodecContext::get_buffer() and not the global one.

It seems that the global one is not in synch when the callback is called.
When MT was in use, non I420 streams were broken.

modules/codec/avcodec/video.c

@@ -976,13 +976,12 @@ static int ffmpeg_GetFrameBuf( struct AVCodecContext *p_context,
     }
 
     wait_mt( p_sys );
-
     /* Some codecs set pix_fmt only after the 1st frame has been decoded,
      * so we need to check for direct rendering again. */
 
-    int i_width = p_sys->p_context->width;
-    int i_height = p_sys->p_context->height;
-    avcodec_align_dimensions( p_sys->p_context, &i_width, &i_height );
+    int i_width = p_context->width;
+    int i_height = p_context->height;
+    avcodec_align_dimensions( p_context, &i_width, &i_height );
 
     if( GetVlcChroma( &p_dec->fmt_out.video, p_context->pix_fmt ) != VLC_SUCCESS ||
         p_context->pix_fmt == PIX_FMT_PAL8 )
@@ -991,7 +990,7 @@ static int ffmpeg_GetFrameBuf( struct AVCodecContext *p_context,
     p_dec->fmt_out.i_codec = p_dec->fmt_out.video.i_chroma;
 
     /* Get a new picture */
-    p_pic = ffmpeg_NewPictBuf( p_dec, p_sys->p_context );
+    p_pic = ffmpeg_NewPictBuf( p_dec, p_context );
     if( !p_pic )
         goto no_dr;
     bool b_compatible = true;
@@ -1037,7 +1036,7 @@ static int ffmpeg_GetFrameBuf( struct AVCodecContext *p_context,
         p_sys->i_direct_rendering_used = 1;
     }
 
-    p_sys->p_context->draw_horiz_band = NULL;
+    p_context->draw_horiz_band = NULL;
 
     p_ff_pic->opaque = (void*)p_pic;
     p_ff_pic->type = FF_BUFFER_TYPE_USER;