avformat/mov: Add simple ACLR atom reading to set the color range of the incomming track for codec's like DNxHD that utilise AVID's proprietary atom.

On input ACLR will be used to set colour range no matter which codec
it is associated with.
No change for when it will be output.

Rework mov_read_extradata function to allow detection of truncated
atom reads by callers.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>