fdkaacdec doesn't support mpegversion 2

Originally reported at https://gitlab.com/goodvibes/goodvibes/issues/107: Goodvibes (a GStreamer-based internet radio player) can't play http://ice2.somafm.com/indiepop-128-aac while running in Flatpak, while it works when running from a Debian package.

Followed-up at https://gitlab.com/freedesktop-sdk/freedesktop-sdk/issues/958. It turns out that the org.freedesktop.Platform//19.08 Flatpak runtime builds ffmpeg with the fdk-aac-free codec. This is not the default AAC codec in ffmpeg, and additionally they strip out the patented bits.

The comment from Michael Catanzaro sums it all: [...] whether it's GStreamer's fault or fdkaac's, or a problem with the way freedesktop-sdk strips the patented bits out of fdkaac (shared with Fedora and Endless)..

More investigation, and it seems that the caps of the fdkaacdec Gstreamer element are the issue. They differ from the caps of avdec_aac for the mpegversion:

# Running in the freedesktop runtime
$ gst-inspect-1.0 fdkaacdec | grep mpegver
            mpegversion: 4

# Running on Debian Buster
$ gst-inspect-1.0 avdec_aac | grep mpegversion
            mpegversion: { (int)2, (int)4 }

The stream I'm trying to play is mpegversion 2.

WARNING: from element /GstPlayBin:playbin0/GstURIDecodeBin:uridecodebin0:
No decoder available for type 'audio/mpeg, framed=(boolean)true, mpegversion=(int)2,
level=(string)2, profile=(string)lc, rate=(int)44100, channels=(int)2, stream-format=(string)adts'.

Is it just a mistake that the fdkaacdec only supports mpegversion 4? Or are there reasons for that?

Thanks.

---

Reproduce the issue in the freedesktop runtime:

RUNTIME_PLATFORM=org.freedesktop.Platform
RUNTIME_SDK=org.freedesktop.Sdk
RUNTIME_VERSION=19.08

flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak install flathub $RUNTIME_PLATFORM//$RUNTIME_VERSION
flatpak install flathub $RUNTIME_SDK//$RUNTIME_VERSION

flatpak run --devel --command=bash --share=network --socket=pulseaudio $RUNTIME_PLATFORM//$RUNTIME_VERSION

# we're now in the flatpak runtime
gst-launch-1.0 playbin uri='http://ice2.somafm.com/indiepop-128-aac'