qtdemux: Parse chan box

This is another variant of providing channel positions.

Also if neither chan nor chnl boxes are found, configure a default channel
layout for raw audio tracks.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/4403

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8928>

subprojects/gst-plugins-good/gst/isomp4/fourcc.h

@@ -310,6 +310,7 @@ G_BEGIN_DECLS
 #define FOURCC_fpcm     GST_MAKE_FOURCC('f','p','c','m')
 #define FOURCC_pcmC     GST_MAKE_FOURCC('p','c','m','C')
 #define FOURCC_chnl     GST_MAKE_FOURCC('c','h','n','l')
+#define FOURCC_chan     GST_MAKE_FOURCC('c','h','a','n')
 #define FOURCC_av01     GST_MAKE_FOURCC('a','v','0','1')
 #define FOURCC_av1C     GST_MAKE_FOURCC('a','v','1','C')
 #define FOURCC_av1f     GST_MAKE_FOURCC('a','v','1','f')