Clean up the handling of mp4s streams. Use the generic esds
descriptor function to extract the palette, instead of hard coding
a wrong magic offset.

Add some more size safety checks when parsing ES descriptors, and
replace magic numbers with the descriptive constants that are already
defined.

Enhance dump output for stsd atoms.

Streams from both bug 712643 and historic bug 568278 now both work
correctly.

Fixes: #712643