avformat/avformat: Add FFInputFormat, hide internals of AVInputFormat

This commit does for AVInputFormat what commit
59c9dc82f4 did for AVOutputFormat:
It adds a new type FFInputFormat, moves all the internals
of AVInputFormat to it and adds a now reduced AVInputFormat
as first member.

This does not affect/improve extensibility of both public
or private fields for demuxers (it is still a mess due to lavd).

This is possible since 50f34172e0
(which removed the last usage of an internal field of AVInputFormat
in fftools).

(Hint: tools/probetest.c accesses the internals of FFInputFormat
as well, but given that it is a testing tool this is not considered
a problem.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/mm.c

@@ -34,6 +34,7 @@
 #include "libavutil/channel_layout.h"
 #include "libavutil/intreadwrite.h"
 #include "avformat.h"
+#include "demux.h"
 #include "internal.h"
 
 #define MM_PREAMBLE_SIZE    6
@@ -189,9 +190,9 @@ static int read_packet(AVFormatContext *s,
     }
 }
 
-const AVInputFormat ff_mm_demuxer = {
-    .name           = "mm",
-    .long_name      = NULL_IF_CONFIG_SMALL("American Laser Games MM"),
+const FFInputFormat ff_mm_demuxer = {
+    .p.name         = "mm",
+    .p.long_name    = NULL_IF_CONFIG_SMALL("American Laser Games MM"),
     .priv_data_size = sizeof(MmDemuxContext),
     .read_probe     = probe,
     .read_header    = read_header,