avformat/matroskadec: Compactify structure

Matroska EBML IDs can be only four bytes long maximally, so it is
natural to use uint32_t for them. By doing this and rearranging the
elements of the MatroskaLevel1Element structure, one can reduce the size
of said structure.

Notice that this field is not read via the generic reading process for
EBML_UINT, so one is not forced to use an uint64_t for it.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -310,8 +310,8 @@ typedef struct MatroskaCluster {
 } MatroskaCluster;
 
 typedef struct MatroskaLevel1Element {
-    uint64_t id;
     uint64_t pos;
+    uint32_t id;
     int parsed;
 } MatroskaLevel1Element;