xiph: handle mixed case comments.

Lowercase or mixed case identifier are legit.
reverts 03c69b44b3ddb9971f34ea4e9bfacc0e23a23a23
xiph: set limit to chapters.

Limits memory size allocation that could have been triggered by
a file, due to chapters array for reordering.
demux: ogg: don't try seeking using bitrate if incomplete.

Results in wrong position if only one track has bitrate info.