isomp4: Fix EC-3 channel count handling in demuxer
requested to merge pgorszkowski-igalia/gstreamer:pgorszkowski/isomp4_fix_eac3_channel_count_handling_in_demuxer into main
Before this change the number of channels in case of EC-3 in caps was reported as 2.
With this change the number of channels for EC-3 is taken from 'EC3SampleEntry Box'. It bases on the specification TS 102 366 (specifically points: 4.4.2.3, F.5, F.6).
Fixes: #2805