Give all anonymously typedeffed structs in headers a name

Anonymous structs cannot be forward declared and have no benefit.

libavformat/swf.h

@@ -64,7 +64,7 @@
 #undef NDEBUG
 #include <assert.h>
 
-typedef struct {
+typedef struct SWFContext {
     int64_t duration_pos;
     int64_t tag_pos;
     int64_t vframes_pos;