lavf: add new struct for data private to generic layer

Trivial for now, will become more useful in future commits, as many
fields from FFFormatContext should not be visible to individual
(de)muxers.

libavformat/avformat_internal.h

@@ -28,6 +28,16 @@
 #include <stdint.h>
 
 #include "avformat.h"
+#include "internal.h"
+
+typedef struct FormatContextInternal {
+    FFFormatContext fc;
+} FormatContextInternal;
+
+static av_always_inline FormatContextInternal *ff_fc_internal(AVFormatContext *s)
+{
+    return (FormatContextInternal*)s;
+}
 
 #define RELATIVE_TS_BASE (INT64_MAX - (1LL << 48))
 

libavformat/options.c

@@ -161,12 +161,15 @@ static int io_close2_default(AVFormatContext *s, AVIOContext *pb)
 
 AVFormatContext *avformat_alloc_context(void)
 {
-    FFFormatContext *const si = av_mallocz(sizeof(*si));
+    FormatContextInternal *fci;
+    FFFormatContext *si;
     AVFormatContext *s;
 
-    if (!si)
+    fci = av_mallocz(sizeof(*fci));
+    if (!fci)
         return NULL;
 
+    si = &fci->fc;
     s = &si->pub;
     s->av_class = &av_format_context_class;
     s->io_open  = io_open_default;