avformat/mux: Set AV_PKT_FLAG_KEY for is_intra_only packet

avformat/mux: Set AV_PKT_FLAG_KEY for is_intra_only packet

The patch will make audio and subtitle packets be marked as AV_PKT_FLAG_KEY.

For audio, it'll caused the audio sample to be sync sample.
To verify ref/fate/movenc results:
1. Get the movenc test data
[lmwang@vpn ffmpeg]$ libavformat/tests/movenc -w && mkdir -p audio_old && mv *.mp4 audio_old_

After applied the patch:
[lmwang@vpn ffmpeg]$ make fate-movenc SAMPLES=../fate-suite
[lmwang@vpn ffmpeg]$ libavformat/tests/movenc -w && mkdir -p audio_key && mv *.mp4 audio_key

2. Get l-smash and build boxdumper
https://github.com/l-smash/l-smash.git

3. dump the box of crc change mp4 and diff -u
[lmwang@vpn ffmpeg]$ ../l-smash/cli/boxdumper --box audio_key/non-empty-moov-no-elst.mp4  > audio_key/non-empty-moov-no-elst.log
[lmwang@vpn ffmpeg]$ ../l-smash/cli/boxdumper --box audio_old/non-empty-moov-no-elst.mp4  > audio_old/non-empty-moov-no-elst.log
[lmwang@vpn ffmpeg]$ diff -u audio_key/non-empty-moov-no-elst.log audio_old/non-empty-moov-no-elst.log
-                default_sample_flags = 0x02000000
-                    independent
-                    sync sample
+                default_sample_flags = 0x01010000
+                    dependent
+                    non-sync sample

4. have checked the change of crc are caused by default_sample_flags
non-empty-moov.mp4, non-empty-moov-elst.mp4,
non-empty-moov-no-elst.mp4, empty-moov.mp4, delay-moov-content.mp4,
empty-moov-second-frag.mp4, empty-moov-second-frag-discont.mp4,
delay-moov-second-frag-discont.mp4, delay-moov-elst-second-frag.mp4
etc

5 For subtitle, it'll effect for tests/ref/fate/binsub-movtextenc and
tests/ref/fate/sub2video, that's expecting result for the subtitle is
marked as keyframe. Below is the checking result of binsub-movtextenc:

[lmwang@vpn ffmpeg]$ ./ffmpeg -i ../fate-suite/sub/MovText_capability_tester.mp4 -map 0 -scodec mov_text -f mp4 -flags +bitexact -fflags +bitexact -movflags frag_keyframe+empty_moov audio_key/binsub-movtextenc.mp4
[lmwang@vpn ffmpeg]$ ./ffmpeg -i ../fate-suite/sub/MovText_capability_tester.mp4 -map 0 -scodec mov_text -f mp4 -flags +bitexact -fflags +bitexact -movflags frag_keyframe+empty_moov audio_old/binsub-movtextenc.mp4
[lmwang@vpn ffmpeg]$../l-smash/cli/boxdumper audio_key/binsub-movtextenc.mp4  > audio_key/binsub-movtextenc.log
[lmwang@vpn ffmpeg]$../l-smash/cli/boxdumper audio_old/binsub-movtextenc.mp4  > audio_old/binsub-movtextenc.log
[lmwang@vpn ffmpeg]$ diff -u audio_key/binsub-movtextenc.log audio_old/binsub-movtextenc.log
.... // the key difference is the flag for sync sample
-                flags = 0x000701
+                flags = 0x000301
                     data-offset-present
                     sample-duration-present
                     sample-size-present
-                    sample-flags-present
                 sample_count = 6
-                data_offset = 188
+                data_offset = 164
                 sample[0]
                     sample_duration = 1570000
                     sample_size = 21
-                    sample_flags = 0x02000000
-                        independent
-                        sync sample
-                        degradation_priority = 0
                 sample[1]
                     sample_duration = 510000
                     sample_size = 2
-                    sample_flags = 0x01010000
-                        dependent
-                        non-sync sample
-                        degradation_priority = 0
                 sample[2]
                     sample_duration = 1690000
                     sample_size = 9
-                    sample_flags = 0x02000000
-                        independent
-                        sync sample
-                        degradation_priority = 0

Suggested-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Suggested-by: Nicolas George <george@nsup.org>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>