libavformat/mxfdec.c: initial support for EssenceGroups

Previous version reviewed-by: Tomas Härdin <tomas.hardin@codemill.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxf.c

@@ -94,6 +94,7 @@ static const struct {
     {AV_PIX_FMT_RGB565BE,{'R', 5,  'G', 6,  'B', 5                         }},
     {AV_PIX_FMT_RGBA,    {'R', 8,  'G', 8,  'B', 8, 'A', 8                 }},
     {AV_PIX_FMT_PAL8,    {'P', 8                                           }},
+    {AV_PIX_FMT_GRAY8,   {'A', 8                                           }},
 };
 
 static const int num_pixel_layouts = FF_ARRAY_ELEMS(ff_mxf_pixel_layouts);

libavformat/mxf.h

@@ -46,6 +46,7 @@ enum MXFMetadataSetType {
     IndexTableSegment,
     EssenceContainerData,
     TypeBottom,// add metadata type before this
+    EssenceGroup,
 };
 
 enum MXFFrameLayout {